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TEXTBOOK  OF  BOTANY 

FOR 

COLLEGES    AND    UNIVERSITIES 


BY    MEMBERS    OF    THE    BOTANICAL     STAFF    OF     THE 
UNIVERSITY   OF    CHICAGO 

JOHN    MERLE    COULTER,    Ph.D. 

PROFESSOR     OF    PLANT     MORPHOLOGY 

CHARLES    REID    BARNES,   Ph.D. 

LATE    PROFESSOR     OF    PLANT    PHYSIOLOGY 

HENRY  CHANDLER    COWLES,   Ph.D. 

ASSOCIATE   PROFESSOR    OF    PLANT    ECOLOGY 


VOL.    I..    PART    II.     PHYSIOLOGY 


NEW  YORK  •:•  CINCINNATI  •:•  CHICAGO 

AMERICAN     BOOK    COMPANY 


Copyright,  1910,  by 
AMERICAN   BOOK   COMPANY. 

Entered  at  Stationers'  Hall,  London. 

a  textbook  of  botany,  vol.  i.,  pt.  n. 
W.  p.    6 


Li**ARY 

Na  c-  s*<*te  College 


PREFACE 

The  study  of  plants  has  assumed  so  many  points  of  view  that  every 
laboratory  has  developed  its  own  method  of  undergraduate  instruc- 
tion. No  laboratory  attempts  to  include  all  the  phases  of  work  that 
may  be  regarded  as  belonging  to  botany ;  and  therefore  each  one 
selects  the  material  and  the  point  of  view  that  seem  to  it  to  be  the 
most  appropriate  for  its  own  purpose.  During  the  last  ten  years 
the  Hull  Botanical  Laboratory  at  the  University  of  Chicago  has  been 
developing  its  undergraduate  instruction  in  botany  to  meet  its  own 
needs.  Freed  from  the  necessity  of  laying  special  stress  upon  the 
economic  aspects  of  the  subject,  and  compelled  to  prepare  students 
for  investigation,  it  seemed  clear  that  its  selection  must  be  the  funda- 
mental facts  and  principles  of  the  science.  Its  endeavor  has  been 
to  help  the  student  to  build  up  a  coherent  and  substantial  body  of 
knowledge,  and  to  develop  an  attitude  of  mind  that  will  enable  him 
to  grapple  with  any  botanical  situation,  whether  it  be  teaching  or 
investigation.  It  has  been  thought  useful  to  present  this  point  of 
view  in  the  present  volume.  The  material  of  course  is  common 
to  all  laboratories,  but  its  selection,  its  organization,  and  its  presenta- 
tion bear  the  marks  of  individual  judgment. 

The  three  parts  of  the  book  represent  the  three  general  divisions 
of  the  subject  as  organized  at  the  Hull  Botanical  Laboratory.  They 
are  felt  to  be  the  fundamental  divisions  which  should  underlie  the 
work  of  most  subdivisions  of  botanical  investigation.  For  example, 
a  study  of  the  very  important  subject  of  plant  pathology  must  pre- 
suppose the  fundamentals  of  morphology  and  physiology  ;  paleobotany 
is,  in  part,  the  application  of  morphology  and  ecology  to  fossil  plants ; 
and  scientific  plant  breeding  rests  upon  the  foundations  laid  by 
morphology,  physiology,  and  ecology.  In  our  selection  for  under- 
graduate instruction,  therefore,  we  believe  that  there  has  been  in- 


12720 


iv  PREFACE 

eluded  the  essential  foundation  for  most  of  the  varied  work  that  is 
included  to-day  under  botany. 

We  recognize  that  the  presentation  of  the  three  great  subjects  here 
included  is  very  compact,  but  the  book  is  not  intended  for  reading 
and  recitation.  The  teacher  is  expected  to  use  it  for  suggestive 
material  and  for  its  organization  ;  the  student  is  expected  to  use  it 
in  relating  his  observations  to  one  another  and  to  the  general  points 
of  view  that  the  book  seeks  to  develop.  There  is  a  continuity  of 
presentation  in  each  part,  so  that  random  selection  may  miss  the 
largest  meaning.  For  example,  in  the  part  on  morphology,  the  thread 
upon  which  the  facts  are  strung  is  the  evolution  of  the  plant  kingdom, 
and  each  plant  introduced  has  its  peculiar  application  in  illustrating 
some  phase  of  this  evolution.  When  certain  groups  are  selected  for 
laboratory  study,  therefore,  the  intervening  text  should  be  read. 

It  is  important  to  call  attention  to  the  fact  that  the  book  has  been 
prepared  for  the  use  of  undergraduate  students.  It  does  not  repre- 
sent our  conception  of  graduate  work,  which  should  include  much 
that  is  omitted  here.  For  example,  the  graduate  student  should 
be  introduced  to  the  original  sources  of  information,  which  would 
involve  an  extensive  citation  of  literature  far  beyond  the  needs  of  the 
undergraduate.  Still  less  has  this  book  been  written  for  our  profes- 
sional colleagues,  who  will  notice  what  they  may  regard  as  glaring 
omissions.  Such  omissions  must  be  taken  to  express  a  deliberate 
judgment  as  to  what  may  be  omitted  with  the  least  damage  to  the 
undergraduate  student.  The  motive  is  to  develop  certain  general 
conceptions  that  are  felt  to  be  fundamental,  rather  than  to  present 
an  encyclopedic  collection  of  facts.  This  purpose  has  demanded 
occasionally  also  a  greater  apparent  rigidity  of  form  in  general  state- 
ments than  is  absolutely  consistent  with  all  the  facts  ;  but  it  was  a 
choice  between  a  clear  and  important  conception  for  one  with  no 
perspective  and  a  contradiction  of  large  truths  by  isolated  facts,  result- 
ing in  confusion.  For  the  same  reasons,  the  extensive  terminology 
of  the  subject  has  been  kept  in  the  background  as  much  as  possible. 
Definitions  usually  are  made  an  incident  to  the  necessary  introduc- 
tion of  terms.  It  is  assumed  that  in  so  far  as  the  definite  application 
of  a  term  may  not  seem  clear,  the  student  will  find  a  compact  defini- 
tion in  the  current  dictionaries. 


PREFACE  V 

For  the  benefit  of  the  teacher  and  of  our  professional  colleagues, 
it  should  be  stated  that  much  attention  has  been  given  to  the  avoid- 
ance of  any  phraseology  that  might  involve  a  teleological  implication. 
It  has  not  been  possible  to  avoid  such  phrases  in  all  cases  without 
introducing  clumsiness  of  expression  or  breaking  the  continuity  of 
some  important  series  of  structures  or  events.  It  should  be  kept  in 
mind,  therefore,  that  all  teleological  implications  of  language  that 
remain  are  disavowed. 

It  seems  hardly  necessary  to  say  that  most  of  the  material  presented 
in  the  book  has  been  worked  over  by  classes  repeatedly.  Some  new 
matter  has  been  developed  incidentally  in  all  the  parts  in  connection 
with  ordinary  laboratory  and  field  work;  and  especially  in  Part  III 
have  many  scattered  observations  and  some  new  points  of  view  been 
included.  There  has  been  no  intention  to  include  any  formal  con- 
tribution, but  merely  to  present  in  general  outline  some  of  the  material 
worked  over  by  undergraduates,  some  of  the  results  of  investigation 
already  published  in  contributions  from  the  laboratory,  and  some  ob- 
servations and  conclusions  that  hardly  seemed  to  justify  separate  pub- 
lication. Provision  has  been  made  for  students  with  more  interest 
or  more  time  than  usual  to  get  a  somewhat  larger  view,  by  including 
in  smaller  type  further  details  of  structure,  additional  illustrative 
material,  and  suggestive  theories.  Most  of  the  illustrations  are  origi- 
nal, in  the  sense  that  they  have  been  prepared  especially  for  this 
book  or  have  appeared  in  our  own  contributions.  Those  that  have 
been  copied  or  adapted  are  credited  ;  the  former  usually  being  indi- 
cated by  "  from,"  the  latter  by  "  after." 

The  three  authors  are  individually  responsible  only  for  their  own 
parts,  and,  while  they  had  the  advantage  of  mutual  criticism,  it  could 
not  be  expected  that  they  would  agree  absolutely  at  every  point. 
This  will  explain  any  lack  of  harmony  that  may  be  discovered  in  the 
three  parts.  A  morphologist,  a  physiologist,  and  an  ecologist  look 
at  the  same  material  from  different  angles,  and  lay  emphasis  upon 
different  features  ;  but  all  their  points  of  view  should  be  included 
in  any  general  consideration  of  plants.  It  is  for  this  reason,  also, 
that  the  parts  contain  a  certain  amount  of  repetition,  which  is  abso- 
lutely necessary  when  the  same  structures  or  functions  are  being 
considered  from  different  points  of  view. 


vi  PREFACE 

The  selection  and  preparation  of  the  illustrations  for  Part  I  were 
under  the  efficient  direction  of  Dr.  W.  J.  G.  Land,  and  most  of  the 
original  drawings  of  the  book  were  made  by  Miss  Anna  Hamilton, 
an  artist  to  whom  great  credit  is  due.  We  owe  certain  original  illus- 
trations to  the  cooperation  of  our  colleagues,  who  are  named  in  con- 
nection with  the  figures;  and  also  some  of  the  drawings  in  Part  III 
to  Miss  Anna  M.  Starr.  In  addition  to  the  mutual  criticism  of  the 
authors,  Dr.  C.  J.  Chamberlain,  Dr.  William  Crocker,  and  Mr. 
George  D.  Fuller  made  helpful  suggestions  in  reading  the  proof. 
For  such  errors  as  remain,  after  all  our  efforts  to  eliminate  them,  the 
authors  themselves  assume  full  responsibility.  In  correcting  them,  we 
shall  welcome  the  help  of  the  wider  circle  of  users  to  whom  the  book 

now  goes. 

JOHN   M.   COULTER. 

CHARLES   R.   BARNES. 

HENRY  C.   COWLES. 
The  University  of  Chicago. 


CONTENTS 


Vol.  I.,  Part  II.     Physiology 


CHAPTER 

Introduction 

I.     The  material  income  of  plants 

1.  The  plant  cell    .     .     .     . 

2.  Diffusion  and  osmosis 

3.  Turgor    and    its   conse- 

quences      

4.  The  permeable  regions  of 

root  and  shoot    .     .     . 
II.     The  material  outgo  of  plants  . 

1.  Transpiration     .     .     .     . 

2.  Exudation  of  water     .     . 

3.  The  movement  of  water 

4.  Other  losses 

III.     Nutrition 

1.  The  nature  of  plant  food 

2.  Photosynthesis  .... 

(1)  The  raw  materials  . 

(2)  The  laboratories      . 

(3)  The  energy    .     .     . 

(4)  The    products   and 

the  process      .     . 

3.  The  synthesis  of  proteins 

4.  Other    ways    of    getting 

food 


(  II  AT  I  KK 


(III) 


IV. 


V. 


5.  The  storage  and  translo 

cation  of  food 

6.  Digestion      .     . 
Destructive  metabolism 

1.    Respiration  .     . 


2.  Fermentations  . 

3.  Waste  products  and  ash 
Growth  and  movement  . 

1.  Growth     .... 

2.  Irritability     .     .     . 

3.  Morphogenic  stimuli 


Nastic  curvatures 
Locomotion  and  stream 


mg 

Turgor  movements 
Tropisms  .... 

(1)  Geotropism     . 

(2)  Thigmotropism 

(3)  Traumatropism 

(4)  Rheotropism 

(5)  Chemotropism 

(6)  Phototropism 

(7)  Other  tropisms  w 

radiant  energy 
The  death  of  plants 


3S8 
397 
403 
403 
409 
412 
417 
417 
426 

435 
442 


444 
4s '" 

458 
459 
469 

472 

473 
4  73 
475 

479 
4S0 


PART    II  — PHYSIOLOGY 


INTRODUCTION 

The  relation  between  the  form  and  structure  of  a  plant  and  its  behavior 
is  very  intimate  and  to  a  large  extent  reciprocal.  Form  and  structure 
in  general  determine  behavior,  and  behavior,  especially  as  it  is  itself 
controlled  by  external  agents,  to  a  great  degree  determines  form  and 
structure.  It  is  not  possible  at  present  to  discover  all  these  reciprocal 
relations,  much  less  to  describe  them  in  terms  of  physics  and  chemistry. 
Nor  is  the  behavior  of  plants  sufficiently  known  to  be  explained  in  these 
terms. 

Morphology,  concerned  with  form  and  structure,  is  particularly  in- 
terested in  how  each  plant  comes  to  be  what  it  is  in  the  short  history 
of  its  own  life  (ontogeny),  and  also  seeks  to  form  a  conception  of  how 
plants  have  come  to  be  what  they  are  in  the  long  course  of  their  history 
since  they  began  to  develop  on  the  earth  (phylogeny).  The  former 
topic  is  clearly  open  to  experimental  study  and  constitutes  the  field  of 
experimental  morphology.  But  the  latter  is  much  less  open  to  experi- 
ment ;  scarcely  at  all,  indeed,  except  for  the  determination  of  the  laws 
of  heredity,  a  field  which  has  been  called  "  experimental  evolution." 
Obviously  such  experiments,  whether  in  the  field  or  laboratory,  cannot  be 
wisely  planned  or  executed  without  a  thorough  knowledge  of  plant 
physiology. 

A  wide  range  of  facts  is  open  also  to  mere  observation,  because  the 
ordinary  changes  in  climate  and  soil,  some  of  which  are  produced  by 
other  plants  and  animals,  affect  the  form  and  structure  of  plants.  This 
field  is  part  of  that  distinguished  from  physiology  proper  as  Ecology 
(Part  III).  Naturally  even  the  most  careful  observations  need  to  be 
confirmed  or  corrected  by  experiments.  Thus  this  portion  of  ecology 
and  experimental  morphology  are  mutually  related,  and  both  really 
form  a  part  of  physiology  in  the  broadest  sense,  and  depend  upon  it. 
Physiology,  in  its  turn,  seeking  to  expound  the  phenomena  of  plant  life 
in  terms  of  matter  and  force,  depends  upon  the  data  of  chemistry  and 

295 


296  PHYSIOLOGY 

physics.  In  certain  directions  present  knowledge  is  almost  Or  quite 
sufficient  to  permit  the  framing  of  physical  and  chemical  explanations. 
In  others  the  data  of  chemistry  and  physics  are  not  yet  adequate  for  this; 
and  in  still  others  it  seems  now  quite  improbable  that  the  phenomena  can 
ever  be  analyzed  in  terms  of  matter  and  force.  It  must  not  be  forgotten, 
however,  that  this  is  the  direction  of  all  recent  advances,  and  that  what 
is  hopelessly  obscure  often  becomes  beautifully  clear  as  some  new  van- 
tage point  widens  the  view. 

In  its  broadest  sense,  then,  plant  physiology  includes  the  study  of 
the  behavior  of  plants  of  all  sorts,  and  of  all  the  ways  in  which  this  is 
affected  by  external  agents  of  every  sort.  On  the  one  hand  it  overlaps 
morphology,  and  on  the  other  it  includes  a  large  part  of  ecology.  In 
this  book,  however,  it  is  restricted  in  the  main  to  a  consideration  of  the 
behavior  of  the  larger  plants,  especially  seed  plants,  though  in  certain 
cases  reference  is  made  to  others.  In  this  part  no  section  on  reproduc- 
tion will  be  found.  That  topic  is  relegated  to  Morphology  (Part  I), 
since  the  purely  physiological  processes  are  relatively  simple,  so  far  as 
known,  and  very  much  alike,  whereas  the  reproductive  organs  are  very 
different  in  different  groups  of  plants  and  are  most  significant  for  their 
morphology.  For  convenience,  also,  the  effect  of  external  agents  on 
plants  is  treated  so  as  to  develop  and  illustrate  general  principles, 
whereas  the  more  extended  account  of  specific  cases  will  be  found  in 
Part  III,  on  Ecology. 


CHAPTER    I— THE   MATERIAL   INCOME    OF   PLANTS 


[.    THE    PLANT    CELL 


An  organ.  —  At  a  glance  one  sees  that  the  body  of  an  ordinary  green 
plant,  such  as  a  bean,  is  segmented,  certain  parts  being  clearly  marked 
off  by  form  from  others.  The  colorless  root  grows  in  the  soil;  the  green 
shoot  grows  in  the  air  and  consists  of  a  distinct  stem  with  lateral  out- 
growths, the  leaves.  Anatomically,  these  parts 
are  members;  but  as  the  work  of  the  plant  is 
distributed  among  them,  each  has  its  functions, 
and  physiologically  each  is  an  organ. 

A  cell.  —  When  one  of  the  organs  of  the  bean, 
such  as  a  leaf,  is  inspected,  one  sees  that  it, 
too,  is  made  up  of  parts,  the  petiole  and  the 
leaflets.  The  latter  are  composed  of  ribs  and 
veins,  with  green  tissue,  or  mesophyll,  between. 
These  parts  also  have  certain  functions  and 
hence  may  be  called  organs.  A  microscopic 
examination  of  the  mesophyll  reveals  that  it  is 
composed  of  minute  bits  of  material  which  has 
come  to  be  known  as  living,  and  is  called  pro- 
toplasm. Each,  individualized,  is  a  protoplast, 
separated  more  or  less  completely  from  its  neigh- 
bors by  membranes  which  it  and  they  have  a  mesophyll  cell  of  a  leaf; 
formed.    The   membrane    and   protoplast   con-  c>  cbloroplast;  n,  nucleus; 

11  v,  vacuole ;  w,  cell  wall. 

stitute  a  cell  (fig.  619). 

Organs  of  a  cell.  —  When  the  protoplast  is  examined  more  closely, 
a  general  translucent  material,  the  cytoplasm,  may  be  distinguished  from 
various  inclusions.  There  are  (a)  many  very  minute  particles,  whose 
nature  is  obscure,  which  tend  to  make  the  cytoplasm  opaque;  (b)  minute 
clear  spaces,  more  fluid  and  sometimes  watery,  the  vacuoles,  many 
of  which  coalesce  as  they  enlarge  with  age,  and  form  a  few  relatively 
very  large  water  spaces  or  only  one;  (c)  a  roundish  nucleus;  (d)  numer- 

297 


298  PHYSIOLOGY 

ous  oval  green  bodies,  the  chloroplasts.  Of  these,  the  nucleus  and 
cbloroplasts,  having  definite  though  only  partly  known  functions,  are 
often  called  organs  of  the  cell. 

The  unit  of  function.  — The  word  "organ,"  then,  is  applied  to  parts 
most  diverse  as  to  size  and  complexity;  it  designates  merely  a  part  when 
its  work  is  thought  of  rather  than  its  structure.  Since  the  various  parts 
of  a  cell  do  not  work  properly  when  separated,  the  cell  may  be  con- 
sidered as  the  unit  of  function,  as  it  is,  for  convenience,  known  as  the 
unit  of  structure. 

Naturally  cells  accustomed  to  association  with  others  do  not  work  properly 
when  separated;  but  there  are  plants  whose  whole  body  is  a  single  cell.  This  fact 
has  influenced  the  conception  of  the  cell  as  a  unit. 

Work  of  the  protoplast.  —  What  a  plant  or  any  part  of  a  plant  can 
do  depends  primarily  upon  the  protoplasts,  since  they  alone  are  com- 
posed of  living  substance;  but  not  all  protoplasts  have  the  same  organs. 
For  example,  the  protoplasts  of  the  leaf  mesophyll,  furnished  with  chloro- 
plasts, can  make  certain  food  when  properly  lighted  and  supplied  with 
carbon  dioxid.  But  in  the  higher  plants  protoplasts  which  lack  these 
organs  cannot  form  food  of  this  kind  under  any  conditions.  The  pro- 
I  toplasts  of  a  tuber,  having  organs  known  as  amyloplasts  (starch-formers), 
are  able  from  suitable  material  to  organize  the  large  starch  grains  that 
constitute  a  form  of  reserve  food  of  much  importance.  These  grains  are 
not  produced  except  by  such  special  organs. 

The  cell  wall.  —  Each  protoplast  jackets  itself  with  a  membrane, 
which  usually  shuts  it  off  completely  from  the  outer  world  and  from  its 
neighbors,  except  for  some  exceedingly  minute  threads  of  cytoplasm 
by  which  it  remains  connected  with  them.  These  threads,  traversing 
the  cell  wall,  persist  from  the  time  of  its  formation.  The  protoplasts  are 
much  hampered  by  these  walls  in  certain  ways,  though  compensating 
advantages  doubtless  accrue.  For  instance,  the  movement  of  the  pro- 
toplast is  restricted,  and  it  cannot  engulf  food  particles,  but  is  limited 
to  the  substances  which  can  dissolve  in  water  and  so  migrate  through  the 
wall.  Thus  the  cell  wall  becomes  a  factor  of  prime  importance  to  the 
plant. 

The  cell  wall  is  the  most  easily  observed  and  striking  part  of  the  cell ;  in  fact  the 
word  itself  commemorates  the  discovery  of  the  empty  chambers  of  cork  and  charred 
wood  which  Hooke  and  Malpighi  and  Grew  saw  (1667-1671)  with  their  primitive 
microscopes,  and  thought  the  fundamental  feature  of  plant  structure. 


THE   MATERIAL   INCOME   OF  PLANTS  299 

Removal  and  alteration  of  the  wall.  — The  cell  wall,  formed  by  the 
protoplast,  is  subject  to  partial  or  complete  removal  by  it.  In  green 
plants  it  is  usually  composed  at  first  of  cellulose;  but  pectic  substances 
early  appear  in  it,  and  with  increasing  age  it  is  subject  to  various  modi- 
fications, which  alter  its  relation  to  water  and  thus  profoundly  affect 
the  conditions  of  life  of  the  protoplast  within. 

One  alteration  to  which  the  wall  is  subject  is  known  as  cutinization. 
because  cutin  is  deposited  or  formed  within  it.  Sometimes,  as  on  the 
outer  face  of  superficial  cells,  this  takes  place  to  such  an  extent  as  to 
form  the  cuticle,  a  layer  which  may  be  loosened  and  removed  entire 
from  the  rest  of  the  wall.  Parts  of  the  outer  wall  adjacent  to  the  cuticle 
may  also  become  impregnated  with  cutin  to  varying  degrees.  The 
cuticle  and  these  cutinized  layers  repel  water,  so  that  a  minimum  only 
is  found  in  the  wall  and  little  can  pass  through. 

By  another  modification  portions  of  the  wall  may  become  gelatinous. 
When  wetted,  they  take  up  great  quantities  of  water  (sometimes  as 
much  as  98  per  cent  of  their  wet  weight)  and  swell  so  enormously  as  to 
lose  altogether  their  usual  firmness. 

Again,  the  wall  may  become  lignified,  a  condition  characteristic  of  the 
walls  of  woody  tissues,  whence  the  name.  Lignified  walls  do  not  swell 
so  remarkably  as  gelatinized  ones,  but  they  allow  water  to  pass  through 
them  with  comparatively  little  resistance. 

Water  of  the  plant.  —  From  what  has  been  said  it  is  evident  that  water 
forms  an  important  part  of  the  cell ;  but  it  is  necessary  to  comprehend  its 
intimate  relations  to  every  part  in  order  to  understand  its  full  significance. 
In  ordinary  land  plants  water  constitutes  always  over  one  half  and  usu- 
ally about  three  fourths  of  their  weight.  Of  the  least  watery  parts,  such 
as  wood,  it  forms  one  half,  and  of  the  most  watery  parts,  such  as  the  pulp 
of  juicy  fruits,  as  much  as  95  per  cent.  In  ordinary  speech  it  is  common 
to  indicate  the  general  character  of  an  object  by  naming  its  most  abun- 
dant component;  as,  a  wooden  table,  a  brick  wall,  wood  and  brick 
being  respectively  the  dominant  but  not  the  only  material  in  the  struc- 
ture. If  the  water  of  the  plant  were  visible  to  the  eye,  distinct  from 
the  other  constituent  materials,  on  the  same  principle  a  plant  might  be 
spoken  of  justly  as  water,  held  in  form  by  other  substances  mingled  with 
it.  This  is  quite  the  reverse  of  the  ordinary  conception,  but  its  essential 
truth  becomes  evident  when  we  consider  not  merely  the  quantity  of 
water  relative  to  other  constituents,  but  attempt  to  picture  the  relations 
of  water  to  the  various  parts  of  the  cell. 


3°° 


PHYSIOLOGY 


Imbibition.  —  When  a  plant  is  placed  in  dry  air,  water  evaporates 
from  it  and  its  various  parts  shrink  and  shrivel.  A  little  shrinkage  occurs 
when  plants  wilt  on  a  hot,  dry  day.  When  water  again  enters  in  suffi- 
cient quantity,  they  swell  and  regain  their  fresh  look.  The  water  may 
even  be  driven  out  entirely  from  some  plants,as  certain  mosses, and  when 
again  wetted,  the  parts  swell  and  regain  partly  or  wholly  their  original 
dimensions.  The  most  obvious  of  these  changes  are  due  to  the  collapse 
or  expansion  of  the  cells;  but  that  they  are  not  limited  to  alterations 
in  the  dimensions  of  the  cells  may  be  shown  by  measuring  a  dry  bit 
of  cell  wall  or  a  dry  starch  grain  under  the  microscope,  and  after 
wetting,  remeasuring  it.  On  examination  it  appears  that  almost  every 
substance  in  the  plant  body  is  capable  of  imbibing  water,  and  of  swelling 
or  shrinking  as  the  proportion  of  imbibed  water  increases  or  diminishes. 
The  smaller  the  quantity  of  water  the  more  difficult,  and  the  larger  the 
amount  the  more  easy  it  is  to  remove  it.  From  the  fully  swollen  gelati- 
nous body  of  a  sea  weed,  Laminaria,  some  water  maybe  extracted  by  the 
pressure  of  the  ringers,  while  the  greatest  pressure  does  not  suffice  to 
squeeze  it  all  out,  and  even  by  heating  it  is  most  difficult  to  remove  the 
last  traces  of  water. 

Theoretical  structures  of  organized  bodies.  —  A  study  of  the  phe- 
nomena of  swelling  by  imbibition,  and  of  the  way  in  which  cell  walls  and 
starch  grains  affect  polarized  light,  permits  some  inferences  either  as  to 
the  form  and  position  of  the  particles,  or  as  to  the  existence  of  strain  or 
tension  between  them,  by  which  they  are  slightly  deformed  or  displaced. 
These  inferences  lead  to  theories  of  the  invisible  structure  of  the  cell 
parts.  The  particles  of  which  wall  and  protoplast  are  composed,  it 
seems  probable,  are  surrounded  by  water.  Whether  these  particles  are 
the  chemist's  molecules,  linked  together  in  a  tense  network,  or  aggre- 
gates of  molecules  (micellae)  having  a  crystalline  form,  which  are 
features  of  the  two  prominent  theories,  is  of  only  remote  significance. 
In  either  case  the  water  between  them  may  increase  or  diminish  in 
amount;  correspondingly,  the  particles  approach  or  recede  from  one 
another.  When  any  water  is  present,  it  forms  a  connected  whole,  how- 
ever irregular  its  distribution  may  be.  The  particles  of  the  swollen  stuff 
also  cohere,  and  remain  so  related  to  one  another  that  when  the  water 
is  all  removed,  they  regain  the  form  they  had  before  it  entered. 

Swelling  and  solution.  —  In  the  recovery  of  the  original  form  is  a 
practical  but  only  a  partial  difference  between  the  behavior  of  merely 
swollen  and  of  dissolved  substances.     In  both  cases  water  wanders  in 


THE    MATERIAL    INCOME   OF   PLANTS 


301 


among  the  particles  and  separates  them  more  or  less  widely.  But  there 
comes  a  limit  to  the  swelling,  and  no  more  water  enters.  If  it  is  removed, 
the  body  regains  its  form  and  the  particles,  presumably,  their  identical 
position.  In  solution  there  is  no  limit  to  separation,  except  by  the 
amount  of  water  present;  and  when  it  is  removed,  the  particles  rearrange 
themselves  in  forms  which  may  be  similar  to  those  of  the  original  body, 
but  are  obviously  not  identical  with  them.  Yet  swelling  may  become 
excessive,  as  when  starch  grains  are  put  into  hot  water  or  alkalies,  and 
after  certain  limits  are  passed  the  swollen  grain  will  not  regain  its  normal 
form.  By  such  transitions  imbibition  merges  almost  insensibly  into 
solution. 

Relations  of  inner  and  outer  water.  —  For  further  understanding  it  is 
useful  to  attempt  to  picture  the  relations  of  the  water  to  the  other  com- 
ponents of  a  young  cell  immersed 
in  natural  water.  The  outside 
water  has  particles  of  many  sorts 
scattered  through  it ;  for  no 
matter  how  pure,  in  nature  all 
water  is  really  a  dilute  solution 
of  various  substances.  The  water 
of  the  cell  wall  has  so  many  par- 
ticles of  cell-wall  stuff  scattered 
through  it  that  nearly  half  the 
volume  is  cellulose;  but  it  is  con- 
tinuous with  the  water  outside. 
The  water  of  the  cytoplasm  and 
of  its  inclusions  is  freer  of  these 
substances,  i.e.  it  is  more  nearly  pure,  because  the  cytoplasmic  particles 
form  only  about  one  fifth  of  the  whole  mass.  This  water,  too,  is  con- 
tinuous with  the  water  of  the  cell  wall,  and  with  that  of  the  solution 
outside.  The  water  of  the  vacuole  is  still  less  encumbered  with  other 
particles,  only  one  or  two  per  cent,  perhaps,  but  these  are  of  diverse 
kinds,  for  the  cell  sap  is  a  solution  of  many  things.  The  water  here  is 
likewise  continuous  with  that  outside  through  the  cytoplasm  and  wall 
(fig.  620). 

Continuity  of  water.  — The  picture  sketched  above  may  be  applied 
to  any  plant  cell  by  modifying  it  to  fit  special  features,  and  may  furnish 
a  working  hypothesis,  crude  though  it  be,  of  the  invisible  structure  of 
organic  bodies  in  general.    This  hypothesis  is  conceived  to  coordinate 


awe         p  t      v 

Fig.  620.  —  Diagram  of  an  imaginary  sec- 
tion through  the  cell  wall  and  protoplast  to 
show  the  possible  relations  of  water  to  the  cell  ; 
a,  outer  water ;  w,  cell  wall ;  e,  ectoplast  ; 
p,  general  cytoplasm  ;  t,  tonoplast  ;  v,  vacuole 
(inner  water);  e,  p,  t,  belong  to  the  protoplast. 


302  PHYSIOLOGY 

the  observed  facts  of  structure  and  of  the  migration  of  substances  into 
the  plant.  The  continuous  cell  wall  determines  that  only  substances 
soluble  in  water  can  enter  the  body.  But  according  to  this  picture  a 
continuous  waterway  is  provided  along  which  water-soluble  substances 
may  travel.  Now  in  order  to  conceive  how  this  migration  occurs,  one 
must  have  a  mental  picture  of  the  behavior  of  watery  solutions.  To 
get  such  a  picture  it  is  necessary  to  bring  to  mind  certain  ideas  of  physi- 
cists regarding  matter  in  its  various  states. 


2.    DIFFUSION   AND    OSMOSIS 

For  convenience,  matter  is  said  to  exist  in  three  states:  gaseous,  liquid, 
and  solid. 

Gases. — One  characteristic  of  gases  is  that  their  particles  tend  to 
separate  and  to  occupy  to  its  utmost  limits  any  receptacle  in  which  the 
gas  is  placed.  If  unconfined  by  impermeable  walls  on  one  side,  they 
form  no  free  surface,  but  show  unlimited  capacity  for  diffusion,  and  their 
particles  may  become  so  dispersed  among  the  other  gases  constituting 
our  atmosphere  as  to  be  unrecognizable  by  any  means  at  our  disposal. 
This  distribution  of  the  particles  is  independent  of  any  mixing  by  mass 
movements,  such  as  those  which  show  as  currents  or  arise  by  jarring  or 
stirring.  On  the  contrary,  it  is  assumed  to  be  due  to  the  energy  of  the 
gas  molecules  themselves,  being  hastened  by  any  means  which  imparts 
energy,  as  by  the  application  of  heat. 

Liquids. — The  molecules  of  liquids  are  much  less  mobile  than  those 
of  gases.  When  placed  in  a  container,  they  shape  themselves  to  it  and 
form  a  free  surface  that  is  horizontal  under  the  action  of  gravity,  from 
which  particles  may  fly  off  as  vapor  into  the  air.  In  volatile  liquids 
this  takes  place  at  ordinary  temperatures  to  such  an  extent  that  the 
process  is  easily  measurable;  in  others,  called  non- volatile,  the  move- 
ment is  too  slight  to  be  observed,  or  is  masked  by  other  changes.  In- 
creasing the  molecular  energy  of  the  liquid,  as  by  heating  it  (unless  it 
dissociates  too  rapidly),  hastens  its  conversion  into  vapor,  which  behaves 
nearly  or  quite  as  a  gas. 

Solids.  — The  particles  of  solids  are  still  less  mobile  than  those  of 
liquids,  so  that  solids  retain  more  or  less  perfectly  their  own  shape,  except 
under  stress.  Some  solids,  like  ice  and  iron,  can  be  liquefied  and  then 
vaporized ;  others,  like  camphor,  may  vaporize  without  passing  through 
the  liquid  state. 


THE   MATERIAL   INCOME   OF   PLANTS 


3°3 


Solution.  —  In  every  state  of  matter  there  exists  a  tendency  of  the 
particles  to  separate,  hampered  more  or  less  by  their  cohesion  or 
mutual  attraction.  Even  very  dense  solids,  such  as  lead  and  gold, 
when  placed  in  contact,  show  intermingling  along  the  line  of  contact, 
though  this  is  so  slow  as  to  be  actually  measurable  only  after  a  long 
time.1  But  when  certain  solids  and  liquids  are  brought  together,  the 
intermingling  occurs  so  speedily  as  to  attract  attention,  and  the  solid 
is  said  to  dissolve  in  the  liquid.  The  liquid  then  is  known  as  the 
solvent,  and  the  former  solid  as  the  solute.  Gases  also  dissolve  in  liquids. 
In  like  manner  when  two  liquids  can  be  mixed  (i.e.  are  miscible),  their 
particles  become  intermingled;  then  one  may  be  considered  as  the 
solvent  and  the  other  as  the  solute;  e.g.  glycerin  and  water.  All  gases 
are  miscible  and  in  all  proportions;  but  not  all  liquids  (e.g.  oil  and 
water),  nor  all  solids  and  liquids.  Otherwise  stated,  when  one  substance 
dissolves  another,  the  two  do  not  always  mix  in  all  proportions;  usually 
there  is  a  limit  to  the  ratio  of  solvent  to  solute,  and  when  the  limit  of 
intermingling  is  reached  (a  condition  called  saturation),  any  excess  of 
the  solute  remains  undissolved. 

Nature  of  solution.  —  It  is  not  necessary  to  the  idea  of  a  solution  that 
the  mixture  should  be  liquid,  though  this  is  the  popular  usage.  A  solid, 
a  liquid,  or  a  gas  may  "  dissolve  "  in  a  solid  and  the  solution  be  a  solid. 
So  a  gas  may  "  dissolve  "  in  a  gas  and  the  solution  be  gaseous.  For  our 
purposes,  then,  a  solution  is  a  mixture  of  substances  so  intimate  that 
they  cannot  be  mechanically  separated;  as,  for  example,  by  filtration. 

The  actual  chemical  state  of  the  substances  is  not  certainly  known.  Moreover, 
by  mingling  finely  divided  but  insoluble  substances,  such  as  lamp  black,  with  a 
solution,  many  particles  of  the  solute  may  be  taken  out,  probably  by  adhesion, 
so  that  this  sort  of  partial  mechanical  separation  is  possible. 

Water  as  a  solvent.  —  Almost  the  only  liquid  which  is  of  much  sig- 
nificance in  plant  life  as  a  solvent  is  water,  and  this  is  capable  of  dis- 
solving more  different  substances  than  any  other  known;  whence  it  is 
said  to  be  the  most  general  solvent  in  nature.  In  water  solutions  the  par- 
ticles of  the  solute  behave  as  those  of  a  gas  ;  they  may  diffuse  to  the  limits 
of  the  solvent,  for  its  boundary  forms  the  only  limit  to  their  movements. 

Natural  solutes. — Water  is  widely  distributed  in  nature,  and  comes 
in  contact  with  many  things;  first,  as  it  falls  in  a  spray  through  the 

'In  an  experiment  in  which  a  rod  of  lead  and  a  disk  of  gold  were  kept  in  contact  for  four  years, 
the  gold  had  diffused  over  7  millimeters  from  the  contact  surface,  in  amounts  appreciable  by 
assaying. 


3°4 


PHYSIOLOGY 


atmosphere,  and  then  as  it  percolates  through  the  soil  and  rocks  or  flows 
over  their  surface.  Hence,  natural  waters  hold  many  solutes,  and  are 
almost  always  in  position  to  acquire  more  if  any  are  removed  by  chemi- 
cal action.  Thus,  the  water  in  arable  soils  contains  everywhere  much 
the  same  amounts  and  kinds  of  mineral  salts;  for,  though  soils  differ 
greatly  in  the  proportion  of  their  constituents,  the  quantities  are  kept 
nearly  constant  by  the  steady  dissociation  of  the  dissolved  minerals, 
by  the  further  solution  of  any  substance  which  has  disappeared  from 
the  water  for  any  reason,  and  by  the  movement  of  solutes  from  one  point 
to  another. 

Diffusion.  —  If  solutes  are  free  to  diffuse  through  the  water  to  its  utmost 
limits,  what  determines  the  direction  and  rate  of  this  movement?  Im- 
agine a  crystal  of  a  soluble  salt  placed  in  a  tumbler  of  water  (fig.  621). 
The  particles  fly  off  from  the  surface  and  become  numerous  in  the  water 
immediately  adjacent.  Here,  freed  partly  from  the  mutual  constraint 
of  the  crystalline  condition,  they  may  be 
conceived  to  be  in  rapid  movement  to  and 
fro,  colliding  often  with  their  fellows  where 
these  are  most  numerous  and  less  often  where 
they  are  fewer.  Hence,  in  regions  towards 
the  crystal,  rebuffs  are  most  frequent;  con- 
sequently the  particles  are  continually  work- 
ing out  into  parts  of  the  solvent  more  and 
more  remote  from  the  crystal  and  the  crowd 
Fig.  621.  — Imaginary  sec-  of  salt  particles,  the  final  result  being  an  equal 

tion  of  a  tumbler  of  water  with  d;stribution  throughout  the  solvent.  The 
a   soluble   crystal,   showing    by  ° 

arrows  the  direction  of  diffusion,  movement  is  from  the  region  where  the  par- 

and  by  dotted  circles  the  lines  of  tjcles  are  most  numerous  to  that  where  they 
equal  concentration.  .  . 

are  less  numerous,  i.e.  from  the  regions  of 
higher  concentration  of  the  solute  to  regions  of  lower.  Or,  since  gas  pres- 
sure is  conceived  to  be  due  to  the  impact  of  the  molecules  on  the  sides  of 
the  container,  and  since  the  solute  behaves  as  a  gas,  it  is  from  regions 
of  higher  to  regions  of  lower  pressure.  For  convenience,  the  ten- 
dency of  solutes  to  diffuse  may  be  called  diffusion  pressure  or  diffusion 
tension. 

Rate  of  diffusion.  — The  rate  of  movement  of  diffusing  particles  of  any 
solute  depends  on  the  difference  in  concentration,  or  the  gradient  of  the 
pressure.  Thus,  when  a  very  soluble  crystal  is  put  into  a  solvent,  the 
rate  of  diffusion  is  at  first  rapid,  because  an  infinitely  high  concentration 


THE   MATERIAL    INCOME    OF    PLANTS 


305 


of  solution  is  adjoined  by  a  zero  concentration;  the  gradient  is  "  steep  " 
because  the  solute  at  infinite  pressure  adjoins  the  pure  solvent  of  zero 
pressure.  But  the  rate  constantly  falls  as  diffusion  progresses,  since  the 
difference  at  any  two  points  is  becoming  less  and  less.  The  rate  is  also 
greatly  influenced  by  temperature,  an  increase  accelerating  and  a  de- 
crease retarding  the  rate,  exactly  as  in  gases.1 

Osmosis.  —  Returning  now  to  the  conception  of  the  relation  of  water 
to  the  plant  cell:  it  might  seem  that,  given  waterways  in  cell  wall  and 
protoplast,  any  solute,  inside  the  plant  or  out,  might  diffuse  in  any  direc- 
tion in  which  its  concentration  is  lower.  And  this  would  be  the  case 
were  there  no  relation  existing  between  the  solutes  and  the  material 
of  the  separating  membranes,  the  cell  wall  and  protoplasm.  These 
modify  the  free  diffusion;  diffusion  through  membranes  or  partitions 
is  distinguished  as  osmosis,  and  the  pressure  which  solutes  may  exert 
on  the  container  is  known  as  osmotic  pressure. 

Unlike  gas  pressure,  to  which  it  is  comparable,  osmotic  pressure  cannot  be  mea- 
sured directly  except  with  great  difficulty.  It  is  calculable  from  the  amount  by 
which  a  solute  lowers  the  freezing  point  and  raises  the  boiling  point  of  the  solvent. 

Permeable  and  impermeable  membranes.  —  Suppose  in  a  closed  glass 
vessel  (fig.  622)  a  glass  partition  divide  A,  pure  water,  from  B,  a  watery 
solution  of  salt.  No  interchange 
of  water  or  salt  between  A  and  B 
is  possible  through  such  a  parti- 
tion, whence  it  is  said  to  be  im- 
permeable. But  if  the  partition 
be  made  of  some  substance  with 
whose  particles  salt  particles  can 
mingle  —  a  substance,  that  is, 
with  which  salt  forms  a  solid  or 
semi-solid  solution — then  the  salt 
particles  which  by  diffusion  reach 
the  A  side  of  the  partition  may  fly  off  thence  into  the  water,  a;  and 
they  will  do  so,  provided  the  attraction  of  the  water  for  the  salt  is 
greater  than  that  of  the  partition  stuff  for  the  salt.  The  nature  of  the 
partition,  then,  determines  whether  any  substance  may  pass  through  it, 
and  of  course  modifies  the  rate  of  its  diffusion. 


Fig.  622.  —  Diagram:  A,  pure  water; 
B,  watery  solution  of  salt,  or  sulfuric  acid; 
p,  portion  of  the  partition  supposed  to  be 
removable  ;  a,  b,  air. 


1  To  avoid  misunderstanding  it  maybe  necessary  to  add  that  under  like  conditions 
each  solute  diffuses  at  a  rate-  peculiar  to  itself. 

C.  B.  &  C.  BOTANY  —  20 


306  PHYSIOLOGY 

This  is  well  illustrated  by  using  air  as  the  partition.  In  fig.  622,  suppose  A  to  be 
jmre  water  and  B  sulfuric  acid,  with  the  impermeable  glass  partition  reaching  only  a 
little  bevond  the  top  of  the  two  liquids,  the  space  above  them  being  filled  with  air. 
Water  (as  vapor)  can  mingle  with  air,  </;  sulfuric  acid  does  not  vaporize  measurably; 
i.r.  the  air,  /»,  is  pra(  tic  ally  impermeable  to  it  but  permeable  to  water.  Water  particles 
therefore  reach  the  b  surface  of  the  air  partition  and  enter  the  sulfuric  acid.  Hence 
the  water  level  in  A  falls  ;  the  acid  level  in  B  rises. 

(  )r,  again:    if  one  place  carefully  in  a  tumbler  (tig.  623)  chloroform,  r,  water, 
w,  and  ether,  r,  the  water  may  be  considered  as  the  partition.     Ether,  being  freely 
soluble  in  water,  diffuses  into  it  and  reaches  the  sur- 
face of  c.    Being  also  soluble  in  chloroform,  it  moves 
on  from   this  surface,  diffusing  in  the  chloroform. 
The  chloroform,  being  only  slightly  soluble  in  water, 
diffuses  into  it  but  slightly.     Finally,  there  remain 
only  two  mixtures:   the  water  saturated  with  ether 
and  chloroform,  and  the  chloroform  saturated  with 
water  and  containing  the  rest  of  the  ether.    This 
experiment  illustrates  not  only  the  solvent  action  of 
the  partition,  but  also  the  way  in  which  the  relations 
of  solubility  between  the  partition  and  the  liquids 
un-am  :     c,    that  it  separates  determine  the  dominant  direction 
water;  e, ether,    of  diffusion. 

The  cell  wall  membrane.  —  Among  the  plant  membranes  through 
which  solutes  pass,  the  cell  wall  seems  to  exercise  little  selective  influence. 
It  is  permeable  to  most  if  not  all  substances  presented  to  it  in  nature. 
For,  externally,  these  are  chiefly  mineral  salts;  and  internally,  the  cyto- 
plasmic membranes  exclude  from  contact  with  it  any  substances  that 
it  also  might  not  allow  to  pass. 

Cytoplasmic  membranes.  — The  protoplast  behaves  quite  differently 
from  the  cell  wall.  It  is  obvious  from  microscopic  examination  that  it 
is  not  uniform  in  structure.  There  is  always  next  to  the  cell  wall  a  deli- 
cate cytoplasmic  layer,  the  ectoplast,  and  each  vacuole  is  bordered  by  a 
similar  layer,  a  tonoplast  (fig.  620). 

Since  a  layer,  apparently  of  the  same  sort,  is  formed  at  the  surface  of  a  fragment 
of  protoplasm  released  by  violence  from  the  cell  wall,  it  seems  probable  that  these 
layers  are  the  result  of  a  change  wrought  in  the  physical  structure  of  the  cytoplasm 
by  contact  with  solutions  of  a  certain  sort,  rather  than  that  they  are  permanent 
organs,  as  they  were  once  held  to  be.  They  are  perhaps  advantageous  in  protect- 
ing the  cytoplasm  from  further  change. 

However  formed,  they  are  limiting  membranes  not  only  in  the  sense 
of  bounding  the  protoplast,  but  also  in  the  sense  of  admitting  and  emit- 
ting some  only  of  the  great  variety  of  solutes  that  come  into  contact  with 


THE    MATERIAL    INCOME   OF   PLANTS  307 

them.  Yet  the  share  of  the  rest  of  the  protoplast  in  this  discrimination 
is  not  to  be  overlooked;  and  since  it  is  impossible  to  analyze  the  action 
of  each  part,  we  may  for  convenience  consider  the  protoplast  as  a  mem- 
brane between  the  vacuole  and  the  outer  world.  But  for  substances  in 
the  protoplast  itself  the  ectoplast  may  act  alone. 

Selective  action.  — The  chemical  composition  of  the  cytoplasm  being 
almost  wholly  unknown  and  doubtless  variable,  no  clear  statement  can 
be  made  as  to  the  mode  of  its  discriminative  action.  It  is  known  only 
that  it  allows  many  substances  to  pass  through  readily  and  debars  olhers; 
and  further,  that  some  substances,  which  are  usually  denied  passage, 
are  permitted  to  pass  under  other  conditions.  These  relations  are  best 
explained  by  the  theory  that  solubility  in  the  membranes  is  prerequisite 
to  osmosis.  If  so,  a  change  in  composition  of  the  cytoplasm  might  ac- 
count for  the  change  in  permeability  that  is  observed  on  occasion. 

It  is  quite  possible  that  local  differences  in  the  composition  of  the  cytoplasmic 
membranes  (a  sort  of  mosaic  composition)  may  permit  the  passage  of  different  sub- 
stances at  different  places. 

Variable  selection.  — The  welfare  of  the  organism  is  largely  dependent 
on  the  discriminative  action  of  the  cytoplasmic  membranes,  for  sub- 
stances requisite  to  food-making  are  allowed  to  enter;  and  foods  are  not 
permitted  to  diffuse  out  and  be  lost.  Chemical  transformations  of  the 
most  varied  kind  occur  within  the  plant,  both  among  the  substances  that 
enter  it  and  are  elaborated  into  foods,  and  also  among  the  foods  that 
are  assimilated.  Of  course  each  change  in  chemical  nature  changes  the 
relations  of  the  substance  to  the  protoplast  and  may  modify  thereby  its 
diffusibility  through  it.  Moreover,  without  known  chemical  change, 
the  mere  presence  of  one  solute  may  greatly  modify  the  behavior  of 
another,  either  by  changing  the  membranes,  or  by  its  direct  influence 
upon  the  other  solute.  With  membranes  capable  of  change,  and  solutes 
capable  of  change,  and  the  almost  unknown  extent  of  the  influence  of 
one  solute  on  another,  the  complexity  of  the  phenomena  of  osmosis 
has  almost  baffled  investigation  hitherto,  but  some  hopeful  progre^  has 
been  made  recently  in  the  discovery  of  factors  determining  the  permea- 
bility of  protoplasm. 

It  cannot  be  too  strongly  emphasized  that  the  "  selection  "  above  described  has 
in  it  no  element  of  choice,  nor  does  it  depend  upon  the  "  needs  "  of  the  plant.  On 
the  contrary,  it  is  purely  physical,  and  depends  solely  upon  the  mutual  relations  of 
the  substances  (membranes  and  solutes)  which  the  conditions  bring  into  contact. 


3o8 


PHYSIOLOGY 


3.    TURGOR    AND    ITS    CONSEQUENCES 

Immigration  of  water.  — The  fact  that  there  are  formed  within  the 
cells  certain  substances  to  which  the  cytoplasmic  membranes  are  nearly 
impermeable,  and  that  they  may  accumulate  to  a  considerable  extent, 
insures  the  entrance  of  water  into  such  cells  either  directly  from  the  out- 
side or  indirectly  from  adjacent  cells  in  which  the  solutions  are  less  con- 
centrated. The  mode  of  this  movement  may  be  conceived  thus.  It  is 
known  that  the  presence  of  any  solute  reduces  the  vapor  pressure  of 
water ;  which,  in  terms  of  current  theory,  means  that  there  are  fewer 
molecules  of  water  per  unit  volume  over  a  solution  than  over  pure  water 
under  the  same  conditions.  Thus  in  fig.  622,  p.  305,  if  A  be  pure  solvent 
and  B  the  watery  solution,  the  actual  pressure  of  water  vapor  in  b  will 
be  less  than  in  a.  If  the  partition  between  a  and  b  be  removed,  the  dif- 
ference in  pressure  would  cause  more  particles  of 
water  vapor  to  move  into  b  in  a  unit  of  time  than 
would  diffuse  in  the  reverse  direction.  If  the  whole 
partition  were  permeable  to  water  and  not  to  the 
solute,  the  same  movement  would  take  place  through 
the  partition;  this  occurs,  it  may  be  conceived, 
because  the  presence  of  the  solute  particles  reduces 
the  internal  pressure  of  the  water,  whose  particles 
thus  diffuse,  in  the  common  fashion,  from  regions  of 
higher  to  regions  of  lower  pressure.  The  conditions 
determining  the  movement  of  the  water  are  created, 
be  it  noted,  by  the  number  and  nature  of  the  solute 
particles. 

Turgidity.  —  As  a  consequence  of  the  migration  of 
water  into  the  vacuole,  the  protoplast  is  forced  out- 
ward against  the  cell  wall,  which,  being  elastic,  is 
stretched  thereby,  unless  the  pressure  is  balanced  by 
an  equal  pressure  from  an  adjoining  cell.  Superficial 
cells,  without  exception  when  healthy,  have  the  free  wall  convex  out- 
ward. The  filamentous  algae  have  the  free  end  often  very  convex 
(fig.  624,  a),  but  the  partitions  between  cells  at  a  little  distance  from 
the  end  are  practically  plane  (fig.  624,  b).  If  the  filament  be  broken 
or  a  cell  dies,  the  adjacent  walls,  previously  plane,  at  once  bulge  out  (fig. 
624,  c)  on  account  of  this  internal  pressure.  When  a  cell  is  surrounded 
on  all  sides  by  those  of  equal  internal  pressure,  its  walls  are  plane. 


Fig.  624.  —  The 
cell  walls  of  a  Clado- 
phora :  a,  a  young  tip 
of  a  filament ;  b,  a 
division  wall  in  the 
middle  of  a  filament ; 
c,  a  division  wall  next 
to  a  dead  cell  (<i). 


THE    MATERIAL    INCOME    OE    PLANTS  309 

The  condition  of  cell  walls  in  a  massive  tissue  may  be  comprehended  clearly  by 
inspecting  a  mass  of  bubbles  such  as  may  be  formed  by  blowing  air  through  a  tube 
into  a  soap  solution.1  The  outer  bubbles  will  have  a  convex  surface,  but  plane 
films  divide  the  air  bubbles  in  the  interior.  Pricking  a  superficial  bubble  gives 
opportunity  for  the  plane  walls  of  those  adjacent  to  it  to  bulge,  because  the  in- 
ternal pressure  is  now  unbalanced. 

A  cell  thus  overfilled  with  water,  with  the  elastic  wall  stretched,  or 
under  strain  and  ready  to  stretch,  is  said  to  be  turgid,  and  the  condition 
is  designated  as  turgidity.  Manifestly,  turgidity  depends  upon  two  fac- 
tors: the  presence  of  a  solute  or  solutes  in  sufficient  amounts,  and  an 
adequate  supply  of  water. 

Turgor  and  osmotic  pressure. — The  pressure  developed  within  the 
cells,  when  an  adequate  amount  of  water  is  at  hand,  may  equal  the  os- 
motic pressure  of  the  solutes  to  which  the  cytoplasm  is  impermeable. 
Obviously,  the  osmotic  pressure  exists,  whether  or  not  it  exhibits  itself; 
it  exhibits  itself  by  stretching  the  elastic  container  only  when  sufficient 
water  can  enter;  this  particular  exhibition  of  it  is  known  as  turgor,2  or 
turgor  pressure.  Thus  within  the  cell  there  exists  both  osmotic  pressure 
and  turgor  pressure;  the  latter  is  a  sort  of  hydrostatic  pressure  depend- 
ent upon  the  former  for  its  existence  and  probably  upon  the  resistance 
of  the  protoplast  and  the  cell  wall  to  filtration  for  its  amount.  It  is 
seldom  likely,  therefore,  to  equal  the  osmotic  pressure. 

Thus,  in  the  cells  of  the  sugar  beet,  the  cane  sugar  alone  has  an  osmotic  pressure 
of  10  or  11  atmospheres;  and  there  arc  certainly  many  other  solutes  which  would 
arid  greatly  to  this.  But  the  turgor  pressure  can  only  reach  a  point  at  which  water 
will  filter  through  the  cytoplasm  and  cell  wall,  and  this  is  probably  less  than  half  the 
osmotic  pressure  of  the  sugar  alone.3 

That  the  osmotic  pressure  is  always  ready  to  produce  turgor  is  shown 
by  the  fact  that  flaccid  cells  placed  in  pure  water  quickly  become 
turgid. 

Plasmolysis.  —  If  a  turgid  cell  is  placed  in  a  solution  more  concentrated 
than  that  within  it,  water  emigrates  from  the  cell,  which  then  becomes 
more  or  less  flaccid.     By  measuring  turgid  cells,  or  making  careful 

1  This  may  be  made  of  a  plain  glycerin  soap.  More  durable  bubbles  may  be  made 
from  this  solution  :  Shaved  while  Castile  soap  10  gm.  by  weight;  warm  water  400  cc. ; 
dissolve.  To  15  parts  by  volume,  add  glycerin  11  parts.  This  will  be  improved  by  al- 
lowing it  to  stand  for  a  week,  cooling  over  night  to  30  C.  and  altering  cold  until  limpid. 

'-'  The  term  is  not  always  thus  restricted;  it  is  often  used  as  synonymous  with  turgidity. 

3  Further  studies  of  this  subject  are  much  needed,  especially  as  the  usual  mode  of  test- 
ing osmotic  pressure  by  plasmolysis  has  been  shown  to  be  faulty. 


3io  PHYSIOLOGY 

camera  drawings  of  them  comparison  after  treatment  with  an  appro- 
priate solution  shows  the  shrinkage  of  the  wall  to  its  unstretched  size. 
If  the  outside  solution  remains  more  concentrated  after  a  loss  of  water 
from  the  cell  just  sufficient  to  permit  the  return  of  the  wall  to  its 
unstretched  condition,  water  continues  to  leave  the  cell.  As  a  conse- 
quence of  the  diminished  volume  of  cell  sap  in  the  vacuole  the  proto- 
plast is  dragged  away  from  the  wall,  if  this  is  rigid  enough  (as  it  often  is) 
to  support  itself;  or  if  not,  the  whole  cell,  wall  and  all,  is  collapsed. 
Usually  only  extreme  shrinkage  from  loss  of  water,  resulting  in  separa- 
tion of  protoplast  from  wall,  is  called  plasmolysis;  but  obviously,  plas- 
molysis  has  two  phases,  inseparable  except  arbitrarily.  It  begins  with 
the  first  emigration  of  water,  and  up  to  the  complete  recovery  of  the  cell 
from  previous  stretching,  it  can  be  detected  best  by  measurement.  In 
its  second  phase  the  further  emigration  of  water  is  made  evident  by  the 
more  or  less  extensive  collapse  of  the  protoplast. 

Rigidity  from  turgor.  — The  emigration  of  water  which  takes  place 
when  a  turgid  cell  is  surrounded  by  a  more  concentrated  solution  is  only 
one  way  by  which  turgor  is  reduced  or  plasmolysis  produced.  The 
evaporation  of  water  may  produce  the  same  effects.  When  a  flexible 
organ,  like  a  leaf  or  a  young  shoot,  loses  water  to  such  an  extent  that  its 
cells  are  no  longer  turgid,  the  parts  bend  by  their  own  weight;  the  edges 
of  the  leaf  and  the  tip  of  the  shoot  droop.  To  the  touch  they  are  less 
rigid  than  before.  This  observation  shows  one  effect  of  turgor.  Thin- 
walled  cells  in  masses,  such  as  form  the  greater  part  of  young  shoots, 
leaves,  and  young  roots,  are  rendered  much  more  rigid  by  the  strains 
set  up  in  the  mass  by  turgor.  Turgor  tensions  in  the  smaller  and  in  the 
less  differentiated  plants,  as  well  as  in  the  younger  parts  of  all  plants,  are 
thus  important  in  maintaining  bodily  form;  whereas  in  the  older  parts, 
especially  of  large  plants,  mechanical  tissues,  characterized  by  thickened 
and  altered  cell  walls,  provide  the  requisite  rigidity. 

Growth  and  turgor. — Besides  its  role  in  maintaining  bodily  form, 
turgor  has  important  relations  to  the  growth  of  cells,  especially  in  the 
phase  when  enlargement  is  the  marked  feature  (see  p.  420).  At  this 
time  water  is  entering  in  relatively  large  amount,  and  turgidity  is  pre- 
requisite to  the  permanent  enlargement.  The  cells  of  flaccid  tissues  do 
not  grow  larger.  Whether  stretching  is  merely  a  mechanical  necessity 
for  such  growth,  or  whether  growth  is  dependent  upon  the  increase  in 
solutes,  which  would  likewise  determine  the  increase  in  turgor,  or 
whether  both  conditions  are  necessary,  is  not  certainly  known. 


THE   MATERIAL   INCOME   OF   PLANTS  311 

Sap  pressure  and  turgor.  — Turgor  plays  an  important  part  also  in 
il  root  pressure  "  (see  p.  3.^)),  by  reason  of  which,  under  certain  conditions, 
water  is  forced  by  the  cells  of  the  cortex  into  the  conducting  tissues, 
whence  it  may  escape  by  filtering  through  the  walls,  or  directly  if  these 
are  cut  or  broken.  Further,  it  is  probable  that  turgor  is  indispens- 
able for  the  excretion  of  water  and  various  solutes  from  superficial  cells. 
But  this  may  be  treated  better  in  connection  with  the  topic  secretion 
(P-337) 

4.    THE    PERMEABLE    REGIONS    OF   ROOT   AND    SHOOT 

Plants  and  water.  — Most  if  not  all  of  the  simpler  algae  and  fungi, 
many  of  the  liverworts  and  mosses,  practically  all  submersed  plants,  and 
the  young  stages  of  even  higher  land  plants  are  readily  permeable  to 
water  and  to  various  solutes  in  every  part  of  the  body.  In  such  case 
they  must  grow  in  water  or  in  very  damp  places.  For,  if  water  may  be 
readily  admitted  over  the  whole  surface,  it  may  be  almost  as  readily  lost 
from  the  whole  surface;  it  will  evaporate  whenever  the  air  in  contact 
with  any  part  of  the  surface  is  not  saturated  with  water  vapor,  and 
this  is  the  usual  condition. 

Terrestrial  plants.  —The  earliest  plants  on  the  earth's  surface,  it  is 
likely,  were  aquatic;  and  in  the  course  of  time  plants  developed  that 
were  adapted  to  temporary  exposure  on  the  shore  rocks  or  along  the 
beaches,  then  to  longer  exposure  and  drier  ground,  until  the  land  finally 
was  occupied  by  plants  which  are  so  constructed  that  they  can  expose 
a  large  part  of  the  body  continually  to  moist  though  unsaturated  air. 
The  deserts  even,  with  only  a  meager  rainfall,  are  by  no  means  barren  of 
vegetation,  but  support  hosts  of  plants,  which  are  able  to  secure  the 
scanty  moisture  from  the  soil  and  to  avoid  in  the  growing  season  ex- 
cessive evaporation  into  the  very  dry  and  often  very  hot  air  to  which  they 
are  exposed.  The  prime  requisite  to  terrestrial  life  is  some  means  of 
reducing  the  evaporation  from  aerial  [tarts  to  an  amount  which  can  be 
replaced  by  the  water  entering  those  parts  of  the  body  that  remain  in 
contact  with  it. 

The  root  system. — The  members  of  the  higher  plants  constantly  in 
contact  with  water  pertain  chiefly  to  the  root  system.1    Of  the  root  sys- 

1  In  sonic  plants  the  underground  stems  an<l  [eaves  (scales)  arc  also  in  contact  with 
water  but  they  arc  almost  impermeable  to  it,  and  hence  may  be  neglected  in  this  connec- 
tion. 


312 


PHYSIOLOGY 


tern,  however,  only  the  younger  parts  are  permeable  to  water,  since  with 
age  the  surface  cells  become  altered,  or  usually  are  underlaid  and  finally 

replaced  by  corky  or  cutinized 
tissues,  whose  walls  are  nearly 
waterproof.  But  as  the  roots  are 
growing  at  the  tips  and  branching, 
there  are  always  young  and  per- 
meable parts. 

Root  hairs.  — The  surface  cells 
of  the  young  root  in  most  land 
plants,  at  a  short  distance  behind 
the  growing  apex,  branch,  sending 
out  tubular  extensions,  the  root 
hairs  (fig.  625),  which  push  their 
way  among  the  soil  particles,  dis- 
placing some  and  being  deformed 
by  crowding  against  others,  to 
which  they  often  adhere  strongly. 
These  root  hairs  increase  5-12-fold 
the  permeable  area  of  the  root,  and 
by  their  size  and   radial   position 

Fir..   625  -Root    hairs   of  lettuce,  with  come  ^   immediate  contact  with 
adherent  soil  grains  (s). —  From  Part  III. 

a  cylinder  of  soil  3-8  mm.  in  di- 
ameter (fig.  626).  They  anchor  the  young  root 
thoroughly,  since  they  adhere  so  firmly  to  the  soil 
particles  that  they  tear  away  from  the  root  when 
that  is  pulled  out  of  even  the  loosest  soil;  and  if 
by  chance  they  come  away,  they  bring  with  them 
the  adherent  grains.  The  root  hairs  are  transient, 
not  living  through  even  one  growing  season.  They 
die  away  on  the  older  parts  of  the  roots,  from 
which  the  hair-bearing  cells  usually  slough  off; 
but  new  hairs  are  being  formed  continually,  during 
the  growth  of  the  root  in  length,  just  behind  the 
advancing  apex.  (See  Part  III,  p.  495,  for  varia-  Fig.  626.— Seedling 
tion  of  root  hairs  and  for  kindred  topics.)  ?f  mustard  ;  a'  grown 

1  '  in  moist  air ;   b,  grown 

Soil. — The  soil,   into  which   roots  clothed  with     in  sand  and  withdrawn 

root  hairs  spread,  consists  of  particles  of  weathered     to  fhow  mass  of  SQl1 

.  ,  ...  ,  .     ,  „         grains  clinging  to  root 

or   comminuted   rock    of    various   kinds,    usually     hairs. —  After  Sachs. 


THE   MATERIAL   INCOME   OF   PLANTS  313 

mixed,  especially  in  the  upper  part,  with  more  or  less  organic  matter, 
the  offal  of  antecedent  animal  and  plant  life.  The  soil  particles 
are  of  various  sizes  and  kinds,  and  the  soil  is  often  named  accordingly. 
Thus  there  are  gravelly,  sandy,  clayey,  and  humus  soils  according  to 
the  amount  of  gravel,  sand,  clay,  or  humus  present.  An  indefinite 
variety  of  mixtures  also  occurs,  as  in  loam,  with  appropriate  descriptive 
names.  The  texture  of  the  soil  depends  chiefly  upon  the  size  of  the 
individual  particles;  but  when  very  fine,  and  especially  when  repeatedly 
wetted  and  dried,  these  often  become  aggregated  into  compound  grains, 
as  is  obvious  in  clay.  The  sort  of  rock  from  which  the  soil  was  made, 
the  size  of  the  particles,  their  state  of  aggregation,  and  the  proportion 
and  character  of  organic  matter,  determine  the  relation  of  water  to  the 
soil,  and  so  the  freedom  and  extent  of  its  movement. 

Soil  water.  —  Of  the  water  which  falls  upon  the  surface  as  rain  all  may 
percolate  into  the  soil,  or  part  may  run  off.  The  character  of  the  soil 
and  of  the  vegetation  on  the  surface,  the  slope,  the  rate  of  precipitation, 
and  the  existent  water  content,  determine  the  fate  of  the  falling  water. 
A  loose  dry  soil  of  level  surface,  a  soil  cover  of  leaves  or  grass,  and  a 
gentle  rainfall,  tend  to  reduce  the  run-off  to  a  minimum.  The  water 
which  percolates  into  the  soil  enters  the  spaces  between  the  soil  particles, 
which  it  fills  more  or  less,  driving  out  the  air  and  adhering  in  the  form 
of  films  to  the  component  particles,  when  it  does  not  fill  the  spaces  com- 
pletely. The  thicker  the  films,  the  less  firmly  the  molecules  more  distant 
from  the  surface  of  the  soil  particles  are  held;  so  that  gravity  suffices 
to  carry  down  to  lower  and  lower  levels  a  certain  amount  of  the  perco- 
lating water.  This  may  drain  away  as  subterranean  streams  or  may 
remain,  saturating  the  soil  at  a  certain  level  and  forming  thus  the  "  water 
table,"  approximately  parallel  to  the  surface  and  at  a  variable  distance 
from  it. 

Capacity  of  soils  for  water.  —  When  all  the  water  that  will  sink  to 
the  water  table  in  a  well-drained  soil  has  drained  out  of  the  upper  regions, 
an  amount  varying  according  to  the  physical  characters  of  the  soil  re- 
mains, adhering  to  the  grains.  The  smaller  spaces  are  still  filled;  the 
larger  contain  bubbles  of  air  which  have  come  in  trom  above  as  the  water 
sank.  If  the  soil  particles  be  very  small  and  close  together,  a  greater 
quantity  of  water  will  be  held  than  in  a  loose,  coarser  soil. 

This  seems  anomalous,  but  as  the  amount  of  water  adhering  to  the  surfaces 
will  be  almost  proportional  to  the  surfaces  themselves,  it  may  easily  be  comprehended 
by  calculating  the  area  of  1000  spheres  each  1  mm.  in  diameter,  which  could 


314  PHYSIOLOGY 

be  packed  into  a  cubic  centimeter,  in  contrast  with  the  area  of  1,000,000  spheres 
each  0.1  mm.  in  diameter  occupying  the  same  place.  In  the  first  case  the  area  would 
be  3141.6  sq.  mm.;   in  the  second,  ten  times  as  much,  or  31,416  sq.  mm. 

In  coarse  soils,  therefore,  such  as  sand,  water  largely  drains  away  ; 
whereas  in  line  soils,  such  as  clay,  it  is  held,  and  it  may  be  so  firmly 
held  as  to  preclude  the  admission  of  more,  once  the  soil  is  saturated; 
whence  a  layer  of  clay  often  forms  a  "  hard-pan,"  in  which  water  col- 
lects as  in  a  basin,  or  over  which  it  runs.  Humus  soils  hold  much  water, 
because  the  particles  of  organic  matter,  besides  being  covered  by  the 
usual  films,  are  not  only  porous,  thus  admitting  water  to  the  interior 
spaces,  but  are  also  able  to  imbibe  it  by  their  very  substance. 

Capillary  ascent  of  water.  —  If  equilibrium  were  momentarily  reached 
among  the  water  films  in  the  soil,  it  would  be  upset  the  moment  any  water 
evaporated  from  the  upper  grains,  for  the  water  film  that  clothed  them 
would  thereby  become  thinner.  This  would  at  once  cause  a  rearrange- 
ment of  the  water  in  all  adjacent  films,  because  the  adjacent  water 
particles  are  pulled  more  strongly  to  the  places  where  the  film  is  thin  than 
they  are  held  where  it  is  thick.  Thus  evaporation  from  the  soil  causes, 
on  the  whole,  an  upward  movement  of  the  water  from  the  deeper  parts 
of  the  soil,  a  disturbance  which  extends  as  far  as  is  permitted  by  the 
resistance  offered  by  the  attraction  of  the  soil  particles  and  by  the  viscosity 
of  the  water.  As  this  effect  may  reach  the  water  table,  the  result  of 
evaporation  is  to  lower  it;  its  level  rises  after  heavy  rain  and  falls  in 
prolonged  drought.  Not  all  the  water  which  enters  the  soil  can  leave  it, 
either  by  drainage  or  evaporation.  Even  if  a  sample  of  the  soil  be  placed 
in  the  air,  very  thin  films  of  water  remain  when  it  is  "  air-dry  "  and  seems 
dry  as  dust.  Only  by  heating  above  ioo°  C.  can  all  moisture  be  driven 
off. 

Migration  of  soil  water  into  roots.  —  When  a  root  penetrates  the  soil 
and  root  hairs  develop  from  all  sides,  the  entire  surface  becomes  clothed 
with  a  film  of  water  just  as  is  the  case  with  the  soil  grains.  When  some 
of  this  water  enters  a  root  hair  or  any  part  of  a  surface  cell,  the  water 
film  becomes  thinner  and  there  takes  place  the  same  sort  of  readjustment 
as  is  produced  by  evaporation  of  water  at  the  surface  of  the  soil,  with  the 
same  general  movement  of  water,  in  this  case  toward  the  root.  In  both 
cases  even  distant  parts  of  the  soil  may  thus  furnish  water  to  make  good 
the  loss.  All  such  movements  of  water,  being  mass  movements  and  not 
diffusion  movements,  involve  the  transfer  of  any  solutes  present  ;  whence 
it  comes  that  solutes  from  a  distance  may  be  brought  into  the  vicinity 


THE    MATERIAL  INCOME   OF    PLANTS  315 

of  a  root  and  may  enter  it  if  the  conditions  permit.  But  inasmuch  as 
the  mineral  solutes  in  the  soil  waters  are  very  similar,  no  matter  what 
the  character  of  the  soil  may  be,  this  is  probably  of  less  importance  to  the 

plant  than  it  would  seem  to  be  at  first  sight. 

Available  water.  —  By  no  means  all  of  the  water  in  the  soil  is  free  to 
migrate  into  the  roots.  There  comes  a  time,  as  the  films  about  the  soil 
particles  become  thinner  and  thinner,  when  the  adhesion  of  the  water 
to  the  soil  grains  is  equal  to  its  diffusion  tension.  Leading  up  to  that 
equilibrium,  it  grows  increasingly  difficult  for  the  plant  to  balance  its 
loss  of  water  by  that  entering  ;  its  eel'  sap  has  become  more  and  more 
concentrated;  and  when  the  outgo  surpasses  permanently  the  income, 
permanent  wilting  usually  ensues  and  often  more  or  less  extensive 
death  of  the  foliage. 

The  water  content  of  a  soil  from  which  no  more  water  can  enter  a 
plant  manifestly  depends  upon  the  plants  concerned,  the  nature  of  the 
soil,  and  other  physical  factors.  It  is  no  fixed  quantity  in  any  case,  and 
at  best  can  be  determined  only  roughly.  To  say  that  it  is  in  sand  less 
than  0.5  per  cent,  in  clay  about  10  per  cent,  in  loam  about  12  per  cent, 
in  humus  about  14  per  cent,  and  in  muck  about  20  per  cent,  is  merely 
to  indicate  the  order  of  magnitude,  "not  to  state  a  fixed  amount.'  These 
figures  become  more  instructive  when  compared  with  the  total  capacity  of 
such  soils  for  water,  which  runs  about  as  follows:  sand,  15  per  cent;  clay, 
50  ner  cent;  loam,  65  per  cent;  humus,  70  percent;  muck,  120  percent. 

Effect  of  roots  on  soil.  —  A  considerable  amount  of  carbon  dioxid 
(( '<  I.  1  and  !<>ss  quantities  of  other  substances  diffuse  from  the  root  into 
the  soil-water  films.  Solution  of  carbonates  is  increased  by  the  pres 
encc  of  C02  in  water,  as  is  shown  by  the  readiness  with  which  a  polished 
marble  plate  may  be  etched  by  roots  traversing  its  surface  and  giving 
off  C02.  Reactions  due  to  other  solutes  which  diffuse  from  the  root,  or 
to  excretions  from  it,  may  determine  the  solution  of  other  sorts  of  soil, 
particles,  and  the  substances  so  dissolved  may  then  enter  the  root.  It 
is  not  known  that  these  changes  so  produced  in  the  soil  are  of  any  con- 
siderable importance  in  plant  life.  Whether  by  diffusion  from  the  roots 
of  live  plants  or  by  the  decomposition  of  dead  roots,  or  by  both,  it  is 
certain  that  various  complex  organic  compounds,  not  yet  fully  known, 
exist  in  soils,  which  may  interfere  seriously  with  the  growing  of  plants 
thereon.  In  certain  soils  the  character  and  quantity  of  these  little 
known  substances  are  so  injurious  that  the  soils  are  almost  sterile 
Even  a  watery  extract  from  them  proves  harmful.     In  such  cases  the 


3' 


PHYSIOLOGY 


soil  can  be  improved  by  mechanical  and  chemical  treatment  designed 
to  remove  or  destroy  the  harmful  compounds.  The  rotation  of  crops 
may  find  partial  explanation  herein;  the  excretions  and  decomposition 
products  of  a  given  crop  may  be  injurious  to  the  same  plants,  but 
less  so  or  not  at  all  to  others.  Even  manuring  may  prove  to  have  its 
value  less  in  the  compounds  put  into  the  soil  than  in  the  improvement  of 
soil  texture  and  the  destruction  of  the  deleterious  compounds  in  it. 

Entry  of  water.  — The  cells  bearing  root  hairs  and  the  adjacent  ones 
are  so  constructed  as  to  facilitate  the  immigration  of  water  and  various 
solutes.  The  cell  walls  are  thin  and  the  protoplast  apparently  forms 
only  a  thin  sheet  over  the  inner  surface,  the  greater  part  of  the  cell 
being  occupied  by  a  huge  sap  cavity.  The  cell  sap  is  usually  a  more 
concentrated  solution  than  the  water  outside;  the  internal  pressure  of 
the  water  is  consequently  less  (p.  308),  and  water  enters,  distending  the 
cell  until  the  elastic  recoil  of  the  stretched  wall  is  sufficient  to  balance 
the  osmotic  pressure  of  the  solutes,  or  to  exude  as  much  water  as  enters. 

Entry  of  solutes.  —  At  the  same  time,  if  any  solutes  to  which  the  pro- 
toplast is  permeable  exist  in  the  soil  water,  but  either  not  at  all  or  in  less 
amount  in  the  cell  sap,  they  will  diffuse  into  the  cell.  But  their  move- 
ment is  as  independent  of  the  movement  of  the  water  as  are  the  condi- 
tions of  such  movement ;  water  and  solutes  move  independently.  If  any 
solute  which  enters  thus  is  not  changed  or  stored  in  the  plant,  i.e.  if 
it  is  not  removed  as  such  from  solution,  it  may  attain  equilibrium  inside 
and  outside  the  plant,  so  that  no  more  enters ;  but  if  it  is  removed  by 
being  chemically  changed  or  by  being  stored,  more  constantly  enters. 

Entry  and  exit  via  roots.  —  The  root  therefore  possesses  permeable 
surface  cells  always  in  contact  with  soil  water,  through  which  water 
and  a  variety  of  solutes,  chiefly  oxygen  and  mineral  salts,  make  their 
way,  under  the  conditions  already  set  forth  regarding  osmosis.  At  the 
same  time,  the  root  permits  through  these  same  surfaces  the  outgo  of  any 
solute  formed  in  the  cells,  to  which  the  cytoplasm  is  permeable,  that 
does  not  exist  at  equal  or  greater  pressure  in  the  soil  water.  It  is  even 
conceivable  that  water  would  pass  out  thus,  were  it  possible  for  the  soil 
to  become  sufficiently  dry.  Artificially  this  can  be  demonstrated;  it 
has  not  been  shown  that  it  occurs  in  nature.  When  the  roots  are  exposed 
to  air,  as  in  transplanting,  especially  if  the  plants  are  to  be  transported 
far,  it  is  necessary  to  guard  against  excessive  loss  of  water  by  evaporation 
from  the  roots;  and  the  quick  drying  of  exposed  roots  is  a  most  obvious 
danger  in  transplanting. 


THE   MATERIAL    ENCOME   OF    PLANT*  317 

Aerial  permeable  regions. — Land  plants  possess  also  certain  per- 
meable regions  on  the  aerial  parts  of- the  shoot.  Small  plants  that  grow 
in  wet  places,  where  the  air  is  very  moist  or  nearly  saturated,  might 
safely  have  all  aerijl  parts  permeable,  because  evaporation  is  slow  and 
the  distance  from  root  to  aerial  surface  short.  Moreover,  spray  or  rain 
falling  on  such  parts  may  enter  there,  as  well  as  soil  water  by  the  roots. 
But  larger  plants  could  not  exist  in  ordinary  dry  air  were  their  permeable 
aerial  surfaces  freely  exposed;  for  if  accessible  to  rain,  the  evaporation 
would  be  dangerously  great.  So  far  as  protection  is  concerned,  large 
plants  with  aerial  shoots  might  thrive  (1)  if  they  were  completely  water- 
proofed, thus  checking  all  evaporation,  or  (2)  if  their  damp  surfaces  were 
shielded  by  drier  partial  coverings,  thus  reducing  evaporation  and 
necessarily  excluding  water. 

Waterproofing  vs.  salts.  — There  seems  to  be  no  a  priori  reason  re- 
lated to  the  necessary  supply  of  water  and  salts  why  the  first  of  these 
alternatives  should  not  have  appeared  in  land  plants.  Structurally,  it 
would  be  quite  possible  to  waterproof  the  aerial  parts  completely,  since 
plants  do  check  water  loss  by  such  means  in  certain  places.  In  such  a 
case,  enough  water  for  other  purposes  might  undoubtedly  enter,  since 
enough  to  supply  the  great  evaporation  now  enters  by  the  roots  alone. 
But,  it  is  objected,  this  would  haVe  prevented  the  intake  of  sufficient 
salts.  As  to  that,  it  is  not  probable  that  stopping  evaporation,  and 
therefore  the  large  inflow  of  water  at  the  roots,  would  interfere  with  the 
supply  of  salts.  This  is  rendered  probable,  because  diffusion  of  solutes 
is  independent  of  the  movement  of  water;  and  to  assume,  as  this  objec- 
tion does,  that  the  solutes  are  carried  along  by  the  entering  water  which 
replaces  that  evaporated,  contravenes  all  that  is  known  about  osmotic 
movement.  Further,  it  is  supported  by  the  observation  that  in  the  rain 
forests  of  Ceylon  (and  doubtless  elsewhere)  there  are  regions  of  luxuri- 
ant vegetation  where  for  months  at  a  time  the  rain  ceases  only  to  be 
replaced  by  a  mist.  In  such  conditions  evaporation  is  almost  impos- 
sible. It  cannot,  therefore,  be  necessary  to  the  adequate  supply  of 
solutes  from  the  soil.  It  is  difficult  or  impossible  to  create  such  con- 
ditions experimentally;  and  ordinary  plants,  accustomed  to  evapo- 
ration, are  so  upset  by  being  grown  in  a  saturated  atmosphere  that 
most  culture  experiments  to  ascertain  the  role  of  evaporation  have 
failed.  The  few  that  have  resulted  in  healthy  development  indicate 
also  that  evaporation  is  not  necessary,  so  far  as  a  supply  of  salts  is 
concerned. 


318  PHYSIOLOGY 

Waterproofing  vs.  gases.  — Though  water  and  salts  might  still  be 
admitted,  a  complete  waterproofing  of  aerial  surfaces  would  exclude  the 
gases  of  the  air,  because  all  substances  must  enter  in  solution.  So,  as  a 
matter  of  fact,  plants  possess  aerial  surfaces  of  large  extent,  freely  per- 
meable, but  shielded  by  covers  which,  while  more  or  less  waterproof, 
are  perforate,  so  that  gases  have  access  to  the  moist  cells  underneath. 
There  is  one  gas,  oxygen,  needed  by  almost  every  plant  for  respiration, 
which  the  terrestrial  plants  can  get  satisfactorily  only  from  the  atmos- 
phere. There  is  another  gas,  carbon  dioxid,  which  is  absolutely  essen- 
tial for  the  food  making  of  green  plants,  and  this  likewise  can  enter  land 
plants  only  from  the  air.  As  the  food  made  by  green  plants  is  the  sole 
supply  for  them  and  for  most  other  living  things,  even  for  man,  and 
further  is  the  chief  source  of  energy  for  doing  the  world's  work,  it  is 
evidently  of  some  importance  that  the  aerial  parts  of  green  plants  should 
expose  wet  surfaces  to  the  air  and  so  make  possible  the  solution  and 
admission  of  oxygen  and  carbon  dioxid. 

Protective  tissues.  — The  admission  of  oxygen  and  carbon  dioxid 
by  the  smaller  plants,  mosses,  liverworts,  and  the  like,  is  made  possible 
by  the  fact  that  the  whole  surface  of  the  body  is  moist  and  therefore 
permeable.  But  the  larger  plants  expose  wet  cell  walls  only  as  the  bound- 
ing surfaces  of  internal  chambers  that  constitute  an  aerating  system, 
shielded  by  a  nearly  waterproof  epidermis  or  a  layer  of  cork  tissue.  ^ 
The  outer  wall  of  the  epidermis  has  its  outermost  layer  so  completely 
cutinized  as  to  constitute  a  continuous  sheet,  the  cuticle;  and  the  sub- 
jacent layers  are  often  infiltrated  with  cutin  to  a  greater  or  less  extent. 
Besides  this,  the  epidermal  cells  not  infrequently  form  wax,  resin, 
and  similar  substances  which  are  secreted  in  granules  or  continuous 
sheets  on  the  outer  wall.  These  substances  all  repel  water,  so  that  only 
minute  amounts  occupy  these  parts  of  the  wall;  consequently  very  little 
can  escape  into  the  air  as  vapor.  On  the  older  parts  of  the  stem,  the 
epidermis  is  at  first  underlaid,  and  later,  sloughing  off,  is  replaced  by 
layers  of  cells,  which,  before  losing  their  living  contents,  impregnate  the 
walls  with  suberin,  so  that  they  become  nearly  impermeable  to  water 
(cork).  Both  these  superficial  waterproof  tissues,  epidermis  and  cork, 
are  perforate  at  numerous  points  (stomata  and  lenticels),  which  com- 
municate with  and  indeed  form  a  part  of  the  aerating  system.  (See  Part 
III  on  cutin  and  cork.) 

Aerating  system.  — This  is  a  network  of  canals  and  spaces,  of  the 
utmost  irregularity  in   land  plants,   and  connected   throughout.    The 


* 


THE   MATERIAL    [NCOME    OF    PLANTS 


319 


passages  are  formed  gradually  among  the  parenchyma  cells  by  partial 
separation  as  they  enlarge.  At  first  all  cells  are  coherent  with  their 
neighbors,  a  necessity  of  the  mode  of  division;  hut  unequal  growth 
and  turgor  produce  strains  which  split  the  common  wall  at  the  corners 
and  sometimes  along  whole  faces  (fig.  627).  In  submersed  water  plants 
the  aerating  system  attains  its  most  marked  development;  huge  canals 
arise  in  the  softer  tissues  of  the  stems  and  leaf-stalks  (tig.  628),  and  in 


Fig.  627.  — Cross  section  of  leaf  of  lily,  somewhat  diagrammafiMp^^fftr  epidermis ;  <•', 
lower  epidermis,  with  stomata, s ,  in  cross  section;  />,  palisad  •;  l^veowi^iml  e',  spongy 

tissue,  with  large  intercellular  spaces  (i)  below  .stoma  (s)  anil  vcflSpa^. —  From  PARI   1. 

other  parts  branched  cells,  the  branches  in  contact  only  by  their  tips, 
leaving  large  space  for  gases.  These  inner  chambers  in  submersed 
aquatics  do  not  communicate  with  the  atmosphere  directly;  they  con- 
tain gases  which  have  come  out  of  solution  in  the  adjacent  cells  and 
constitute  an  internal  atmosphere  into  which  gases  may  diffuse  or  from 
which  gases  may  migrate  into  the  living  cells  (of  course  in  solution) 
(See  further,  Part  III,  p.  551.) 


3  20 


PHYSIOLOGY 


Fig.  628.  —  Cross  section  of  stem  of  Myri- 
ophyllum,  with  air  canals.—  From  Part  III. 


Stomata.  — The  aerating  system  of  the  terrestrial  plants,  and  of  water 
plants  not  normally  completely  submersed,  communicates  with  the  at- 
mosphere freely,  because  certain 
cells  of  the  epidermis,  predeter- 
mined by  the  mode  of  their  de- 
velopment, break  apart  through 
the  central  portion  of  their  last- 
formed  division  wall.  As  imme- 
diately beneath  them  an  air  space 
of  some  size  develops,  this  estab- 
lishes a  passage  to  the  outer  air. 
These  two  crescentic  cells  of  the 
epidermis  are  the  lips  of  a  mouth- 
like slit  called  a  stoma;  the  two 
lips  are  called  guard  cells  (fig.  629). 
The  guard  cells  differ  from  other 
epidermal  cells  in  their  crescentic 
form  and  smaller  size,  and  in  having  chloroplasts  which  are  usually 
absent  from  other  epidermal  cells.  Their  walls  are  also  peculiarly  and 
unequally  thickened  (see  also  Part  III,  figs.  794-806).  Their  turgor 
variations,  the  unequally  thick  walls,  and  their  position  with  respect  to 
the  adjacent  cells  make  them  change 
shape,  with  increasing  turgor  becom- 
ing more,  arcuate  and  with  lessening 
turgor  straighter.  The  effect  of  these 
changes  is  to  widen  or  narrow  the  slit 
between  them,  so  making  more  free  or 
restricted  the  passage  of  gases  either 
by  flow  or  diffusion. 

Size  and  number  of  stomata.  —  A 
stoma  is  very  minute;  the  area  of  the 
pore  when  open,  in  thirty-seven  sorts 
of  cultivated  plants,  averages  0.000092 
sq.  mm.  But  their  great  number  on 
those  organs  (such  as  leaves)  in  which 
the  admission  and  exit  of  gases  is 
most  free,  makes  up  for  their  small  size.  Both  features  will  be  grasped 
better  by  this  statement :  in  an  area  equal  to  that  of  the  dot  here 
printed  (•),  there  are  on  the  under  side  of  the  apple  leaf  over  1400 


Fig.  629. — ■  Stoma  of  Scdum  ;  a,  a,  a, 
first  wall,  cutting  off  mother  cell  of  stoma ; 
b,  b,  b,  second;  c,  c,  c,  third;  d,  d,  fourth; 
e ,  e ,  final  wall ;  the  latter,  forming  the  two 
guard  cells,  g,  g,  partially  splits  to  form 
the  slit  (s);  1,  2,  3,  subsidiary  cells. 


THE   MATERIAL   INCOME   OF   PLANTS 


321 


stomata,  and  on  the  under  side  of  the  olive  leaf  about  3700.  The 
following  table  (after  Weiss)  shows  the  numbers  per  square  millimeter 
in  various  common  plants. 


Name  of  plant 

Number  of 
stomata 

Name  of  plant 

Number  of 

STOMATA 

Upper 
side 

Under 
side 

Upper 
side 

Under 
side 

Olca  curopaea  (olive)   . 

Castalia  odorata  (white 
water  lily) 

Helianthus  animus  (sun- 
flower)     

Syringa  vulgaris  (lilac) 

Solarium  Dulcamara  (bit- 
tersweet)        

Pisum  sativum  (pea)     .     . 

Ficus  elastica  (rubber 
plant) 

O 

460 

175 
0 

60 
1  or 

0 

625 

O 

32  5 
330 

26s 
216 

145 

Zca  Mays  (Indian  corn)  . 

Bctula  alba  (white  birch) 

Berberis  vulgaris  (bar- 
berry)   

Populus  deltoides  (Cot- 
tonwood)   

Pinus  Strobus  (white 
pine) 

A  vena  saliva  (oats)  .     . 

Lilium  bulbiferum  (tiger 

Hiy) 

94 
0 

O 

89 

142 
48 

O 

158 
237 

229 

I31 

0 
27 

62 

So  far  as  plants  have  been  examined,  it  appears  that  a  large  majority 
of  mesophytes  have  less  than  200  stomata  to  the  square  millimeter, 
and  a  fair  average  is  perhaps  150.  (See  Part  III,  p.  556,  on  variations 
in  the  structure  and  distribution  of  stomata,  and  the  causes  thereof.) 

Transpiration.  —  Since  the  intercellular  spaces  arc  bounded  by  moist 
cell  walls,  freely  permeable  to  water,  they  are  always  filled  with  air 
which  contains  more  or  less  water  vapor.  This  vapor  diffuses  through 
the  stomata  into  the  drier  outer  air,  and  being  lost  from  the  plant  will 
be  replaced  in  whole  or  in  part  by  water  entering  the  root.  At  the 
same  time,  since  the  walls  of  the  epidermal  cells  contain  a  little  water, 
some  evaporation  takes  place  directly  from  them.  The  total  evapora- 
tion of  water  under  these  conditions  is  designated  as  transpiration 
(see  p.  323). 

Exit  but  no  entry  for  water.  — The  aerial  parts  are  constantly  losing 
water  because  they  are  permeable  ;  at  the  same  time,  there  is  practically 
no  opportunity  for  the  admission  of  water,  even  when  such  parts  are 
deluged  by  it.  Ordinarily  rain  comes  into  contact  only  with  a  nearly 
waterproof  surface,  the  cuticle.  It  cannot  easily  penetrate  the  minule 
stomata,  even  when  they  occur  on  the  upper  surface  of  leaves,  for  there 


322 


PHYSIOLOGY 


are  usually  some  special  substances  or  structures  that  repel  water ;  and 
so  it  does  not  come  into  contact  with  the  wet  and  permeable  walls  of  the 
internal  cells.  Here  then  is  an  arrangement,  not  found  elsewhere  in  the 
plant,  by  which  water  may  leave  the  body  rather  freely,  yet  practically 
cannot  enter  it  when  conditions  are  reversed. 

It  may  be  assumed  that  there  may  enter  the  cuticle,  when  wet,  amounts 
of  water  corresponding  to  those  that  evaporate  from  it  when  dry.  The  re- 
vival of  wilted  plants  after  the  foliage  is  sprinkled,  however,  is  due  chiefly  to 
checking  the  evaporation ;  yet  the  trifling  amount  of  water  entering  tends  to 
the  same  result. 

Entry  and  exit  of  gases.  — The  aerial  parts  facilitate  the  entry  and 
exit  of  gases.  The  external  atmosphere  communicates  freely  with  the 
internal  atmosphere  of  the  intercellular  spaces  by  way  of  the  stomata. 
Any  oxygen  or  carbon  dioxid  in  the  air  of  the  intercellular  spaces  may 
dissolve  in  the  water  of  the  cell  walls  and  then  migrate  into  the  adjacent 
cells,  if  the  pressure  of  these  solutes  is  less  in  the  cells  than  in  the  internal 
atmosphere.  In  like  manner  either  may  diffuse  into  the  internal  at- 
mosphere when  the  reverse  conditions  exist.  The  solubility  of  C02  and 
02  in  water  under  like  conditions  is  very  unequal,  the  former  being  about 
30  times  as  soluble  at  ordinary  temperatures  as  the  latter.  The  rate  of 
diffusion  is  also  unequal.  The  quantity  of  each  used  or  produced  by  the 
plant  likewise  differs.  These  factors  all  play  a  part  in  determining  the 
amount  of  gas  which  enters  or  leaves.  As  the  composition  of  the  internal 
air  fluctuates  on  account  of  subtraction  or  addition  of  COa  or  02,  a  dif- 
ference is  created  between  the  internal  and  external  atmosphere,  which 
leads  at  once  to  diffusion  through  the  stomata  in  a  direction  determined 
by  the  existing  inequality  in  pressure  of  either  gas.1  Nitrogen,  the 
only  other  considerable  constituent  of  air,  is  neither  used  nor  produced; 
hence  practical  equilibrium  between  the  N2  of  the  air  and  the  N2  in 
solution  in  the  plant  is  early  attained,  and  this  equilibrium  is  scarcely 
disturbed  thereafter. 

In  submersed  plants  the  oxygen  and  carbon  dioxid  are  dissolved 
in  the  water  and  find  admission  at  any  permeable  surface,  like  other 
solutes. 

1  Further  discussion  of  the  r61e  of  these  gases  will  be  found  in  the  sections  on  Photo- 
synthesis (p.  363)  and  Respiration  (p.  403). 


CHAPTER    II  — THE    MATERIAL    OUTGO    OF   PLANTS 
i.    TRANSPIRATION 

The  term  transpiration.  —  Frequent  reference  has  already  been  made 
to  the  most  important  outgo  of  material  from  the  plant  body  —  the  water 
evaporated  from  the  aerial  parts.  This  was  long  ago  called  transpira- 
tion, after  the  analogy  of  the  exhalation  of  water  vapor  from  the  lungs, 
with  whose  movements,  however,  it  has  nothing  in  common.  It  is 
considered  by  many  to  be  a  function  of  the  aerial  parts,  something 
which  they  actively  do,  in  which  case  a  special  name  would  be  quite 
appropriate.  It  is  better,  however,  to  look  upon  it  as  a  process  in  which 
they  are  passive.  In  this  case  evaporation  is  no  more  a  "  function"  of 
a  wet  leaf  than  it  is  of  a  wet  towel,  and  the  need  of  a  special  term  is  less 
evident.  Yet  the  word  is  convenient  as  a  short  form  for  the  expression, 
the  evaporation  of  water  from  live  plants. 

Evaporation.  —  When  a  dish  of  water  is  exposed  to  air  which  contains 
less  water  vapor  than  it  can  hold,  more  water  particles  will  fly  off  into 
the  air  in  a  given  time  than  will  fall  into  the  water  from  the  air;  hence 
the  volume  of  liquid  will  be  diminished;  the  water  evaporates.  The 
rate  of  evaporation  is  determined  by  the  temperature  of  the  water,  the 
temperature  and  pressure  of  the  air,  and  the  relative  amount  of  water 
vapor  in  the  air  (humidity).  Decreased  humidity,  higher  temperature, 
or  lower  pressure  increases  the  rate  of  evaporation,  and  vice  versa.  The 
presence  of  any  solutes  in  the  water  retards  evaporation.  Likewise 
water  adherent  to  any  substance,  or  imbibed  by  it,  is  held  there  and 
evaporates  less  readily  than  if  in  contact  with  water  particles  only. 
Thus  the  water  evaporates  from  a  dish  of  wet  sand  or  from  a  wet  towel 
or  sponge  more  slowly  than  from  an  equal  surface  of  free  water. 

since  the  actual  exposed  surface  may  lie  greatly  increased  by  spreading  out  the 
water  over  sand  grains  or  linen  fibers,  the  evaporation  from  a  given  area  of  the 
material  is  not  comparable  with  that  from  an  equal  area  of  water. 

Because  the  evaporation  from  a  green  leaf  and  that  from  a  like  area  of  water  are 
not  equal  is  no  reason  for  giving  a  special  name  in  the  evaporation  from  leaves, 
as  has  been  urged.     If  it  were,  we  should  need  one  term  for  evaporation  from  a 

.123 


324 


PHYSIOLOGY 


towel,  another  for  evaporation  from  a  sponge,  etc.,  for  the  rate  varies  always  accord 
ing  to  the  material  with  which  the  water  is  in  contact. 

Adhesion.  — The  water  which  is  part  of  a  plant  body  adheres  to  the 
particles  of  cell  wall,  cytoplasm,  and  its  inclusions,  and  is  held  with  un- 
equal tenacity  according  to  the  amount  of  each  substance  and  its  rela- 
tion to  water.  As  a  rule,  the  greater  the  proportion  of  water  in  any  sub- 
stance, the  less  firmly  it  is  held.  The  attractions  between  the  water 
particles  and  plant  substance  are  altered  when  the  plant  is  "  killed." 
Thus,  if  a  living  and  a  dead  leaf  be  placed  side  by  side  in  dry  air,  the 
dead  leaf  loses  its  water  much  more  rapidly  than  the  living  one,  and 
shrivels  in  a  few  hours.  Probably  this  is  in  large  part  due  to  changes 
that  the  cytoplasm  undergoes,  which  we  call  death;  but  these  cannot  be 
accurately  described,  beyond  certain  gross  visible  changes  that  do  not 
help  us  to  understand  the  matter. 

Cytoplasmic  changes.  — There  are  many  changes  that  the  cytoplasm 
may  undergo,  which,  though  not  visible, occur  in  the  course  of  daily  living. 
The  nature  of  these  changes  is  not  known,  and  the  precise  way  in  which 
they  affect  water  loss  is  not  known.  Some  of  them  may  be  produced 
by  the  very  diminution  of  the  water  content  itself  and  thus  at  any 
moment  may  operate  to  alter  suddenly  the  rate  of  evaporation. 

A  somewhat  analogous  action  is  known  in  the  case  of  a  number  of  salts  which 
form  hydrates  with  variable  quantities  of  water.  Thus,  copper  sulfate  forms  a 
pentahydrate,  a  trihydrate,  and  a  monohydrate.  In  drying  at  500  the  pentahydrate 
(CUSO4,  5H2O)  maintains  a  vapor  pressure  of  47  mm.  (mercury)  as  long  as  any 
pentahydrate  remains;  then  the  vapor  pressure  suddenly  drops  to  30mm.,  that  of 
the  trihydrate  (CUSO4,  3H2O).  With  further  desiccation  it  again  suddenly  falls, 
as  soon  as  the  trihydrate  is  all  decomposed,  to  4.5  mm.,  the  vapor  pressure  of  the 
monohydrate  (CuS04,  H20),  and  there  it  remains  until  all  the  water  is  driven  off. 
In  this  case  there  would  be  at  each  point  a  sudden  fall  in  the  rate  of  evaporation. 
Just  such  sudden  alterations  have  been  observed  in  transpiration. 

Regulation.  —  To  say  that  the  living  protoplast  "  regulates  "  the  loss 
of  water  from  a  plant  is  only  to  say  that  as  the  nature  of  the  living 
material  may  change,  its  water  relations  change,  and  the  rate  of  evapo- 
ration changes  in  consonance.  But  this  is  not  "  regulation  "  in  the  sense 
of  adjusting  the  loss  to  the  income,  so  that  no  harm  may  come  to  the 
plant.  It  is  regulation  only  in  the  sense  that  the  crystal,  when  heated, 
"  regulates  "  the  loss  of  its  component  water.  In  both  cases  evaporation 
becomes  increasingly  difficult,  and  for  the  plant  this  may  avert  death 
from  excessive  water  loss. 


THE   MATERIAL   OUTGO   OF   PLANTS  325 

Influx  of  water.  — Transpiration  has  been  called  a  function  because 
it  creates  a  current  of  water  through  the  plant,  whi<  h  was  falsely  sup- 
posed to  sweep  in  with  it  the  needful  mineral  salts.  But  it  is  impossible 
to  reconcile  this  conception  with  present  ideas  of  osmotic  movement. 
The  only  condition  under  which  more  water  can  enter  is  when,  by  the 
concentration  of  solutes  in  the  plant,  the  internal  pressure  of  the  water 
of  these  solutions  has  been  reduced;  and  this  is  precisely  the  tendem  yof 
evaporation.  If  the  water  and  plant  substance  were  in  equilibrium, 
evaporation  from  aerial  parts  would  upset  this  equilibrium  by  reducing 
the  amount  of  water,  which  would  be  replaced  by  the  entrance  of  water 
at  any  permeable  region  in  contact  with  it.  Hut  this  would  by  no  means 
furnish  an  adequate  reason  for  the  entrance  of  any  solute  which  was  in 
equilibrium  before  evaporation  took  place.  On  the  contrary,  by  con- 
centration of  the  solution,  the  tendency  would  be  in  the  opposite  direc- 
tion; the  solutes  to  which  the  protoplasts  were  permeable  would  emigrate. 
And  the  mineral  salts  in  question,  being  admissible  by  hypothesis, 
would  do  this.  Transpiration,  therefore,  may  occasion  an  influx  of 
water,  but  not  of  salt ;   indeed  it  might  easily  cause  an  outgo  of  salts. 

Transpiration  and  salts. — Transpiration  has  been  called  a  function, 
also,  because  it  was  supposed  to  be  useful  in  concentrating  the  dilute  solu- 
tions of  salts  brought  up  to  the  leaves.1  That  evaporation  of  water  from 
the  leaves  would  tend  to  do  this  is  true,  of  course.  But  the  loss  of  water 
is  at  once  compensated,  under  favorable  conditions,  by  the  entry  of  more 
water,  and  the  solutions  are  again  diluted.  If  equilibrium  were  assumed 
for  the  moment,  then  the  disturbance  of  equilibrium  by  evaporation 
would  determine  a  movement  of  water  to  readjust  it,  and  the  solution 
would  again  be  brought  to  the  same  concentration.  Were  a  liter  of  water 
containing  a  gram  of  cooking  salt  set  on  the  fire  to  boil,  and  were  pure 
water  added  as  fast  as  it  boiled  away,  no  concentration  of  the  salt  solu- 
tion could  occur.  But  if  salt  solution  were  added  as  water  evaporated, 
the  concentration  of  the  salt  would  be  constantly  increasing.  This  idea 
of  the  concentration  of  dilute  solutions  in  the  leaves  by  evaporation  in- 
volves, therefore,  the  same  assumption  as  the  other  "  function  "  assigned 
to  transpiration;  namely,  that  water  carries  along  with  it  the  dissolved 
salts,  as  a  river  current  sweeps  along  suspended  mud.  But  this  is  a  mere 
assumption,  and  contradicts  both  theory  and  observation  of  osmotic 
movement. 

1  One  popular  book  for  children  even  speaks  of  leaves  as  the  plant's  "kitchens,"  where 
the  thin  "soups"  arc  boiled  down. 


326  PHYSIOLOGY 

A  possible  advantage. — There  is  only  one  region  in  the  plant  where 
solutes  may  move  with  the  water;  that  is,  where  solutions  move  as 
a  whole,  namely,  in  the  conducting  tissue,  which  extends  from  root  cortex 
to  leaf  cortex.  But  solutions  cannot  enter  this  tissue  in  the  live  plant 
without  first  passing  through  several  live  cells  of  the  cortex,  where  os- 
motic movement  only  is  possible;  nor  can  they  usually  reach  the  evap- 
orating surface  of  a  leaf  (the  wet  walls  of  the  aerating  passages)  without 
passing  several  live  cells,  where  again  the  solutes  and  water  must  move 
independently.  (See  movement  of  water,  p.  341.)  It  is  conceivable 
that  the  relatively  rapid  movement  of  solutions  along  this  portion  of  the 
path  from  root  to  leaf  may  be  advantageous  to  the  plant  by  placing  a 
greater  supply  of  salts  within  reach  of  the  leaves  ;  but  there  is  no  proof 
that  plants  depend  on  this  arrangement  for  an  adequate  amount  of  salts. 
Moreover,  this  is  rendered  improbable  by  the  fact  that  many  plants  grow 
most  luxuriantly  with  practically  no  transpiration  for  months  at  a  time 
to  set  up  such  a  stream  of  solutions  along  the  conducting  tissue. 

A  menace  to  life.  — Transpiration,  far  from  being  a  function  of  plants, 
is  an  unavoidable  danger.  That  it  is  a  danger,  a  real  menace  to  life, 
is  almost  a  matter  of  common  observation.  Millions  of  plants  perish 
annually  because  the  outgo  of  water  is  greater  than  the  income.  A 
loose  soil  and  an  exposed  situation,  sudden  extreme  evaporation  due  to 
a  hot  dry  wind  and  a  blazing  sun,  or  prolonged  drought,  are  causes  of 
death  only  too  well  known  to  farmers  in  some  regions.  Scarcely  a  plant 
escapes  the  loss  of  some  parts  by  reason  of  shortage  in  the  water  supply; 
and  in  temperate  regions,  with  the  average  rainfall  (say  100  cm. 
annually),  few  plants  attain  the  development  of  which  they  are  capable 
with  a  larger  water  supply.  The  luxuriant  weed  of  well-watered  ground 
compared  with  the  same  weed,  meager  and  dwarfed  on  the  dry  wayside, 
illustrates  what  a  menace  to  life  and  vigor  is  the  evaporation  from 
plants. 

Transpiration  and  growth.  — There  are,  of  course,  other  causes  of 
stunting  and  meager  development  than  transpiration.  If  some  of  these 
operate  to  reduce  vigor  and  growth,  transpiration  is  affected  thereby. 
In  fact,  growth  and  transpiration,  in  seedlings  at  least,  seem  to  be  recip- 
rocally related,  and  the  one  varies  directly  as  the  other,  when  an  ample 
supply  of  water  is  available,  as  in  a  water  culture.  It  is  not  improbable 
that  a  like  relation  exists  under  these  conditions  in  mature  plants. 

Transpiration  unavoidable. — Dangerous  as  transpiration  is,  it  is 
unavoidable,  because  moist  cell  walls  must  be  exposed  to  permit  solu- 


THE   MATERIAL   OUTGO   OF    PLANTS  327 

tion  and  entrance  of  the  gases  absolutely  indispensable  to  life.  To  be 
sure,  the  outer  walls  of  the  surface  cells  arc  relatively  dry,  esp©  ially  in 
plants  of  dry  regions,  where  water  loss  is  to  be  reduced  to  a  minimum. 
Of  the  total  water  lost  scarcely  more  than  20  per  cent,  and  as  little  as  3 
per  cent,  escapes  through  the  epidermis.  This  evaporation  is  sometimes 
distinguished  as  cutii  ular  transpiration.  The  remaining  80-97  Per  cent 
diffuses  through  the  stomata  and  constitutes  stomatal  transpiration.  The 
efficiency  of  this  arrangement  in  reducing  transpiration  and  yet  admit- 
ting gases  freely  is  more  obvious  when  one  observes  that  the  actual 
evaporation  surface  —  i.e.  of  the  cells  bounding  the  intercellular  spaces 
—  is  several  times  that  of  the  leaf  itself. 

The  place  of  maximum  rutirular  evaporation  has  hern  shown  In  some  leaves  to 
be  that  part  of  the  outer  wall  of  the  epidermis  where  the  side  walls  abut.  In  these 
cases  water  of  imbibition  is  more  abundant  there  than  elsewhere. 

It  is  impossible  to  determine  the  actual  surface  exposed  in  the  very  irregular  air 
passages.  If  a  leaf  1  mm.  thick  had  an  epidermis  0.1  mm.  thick  of  coherent  cubical 
cells  on  each  face,  and  if  the  remaining  cells  were  spheres  each  0.1  mm.  in  diameter, 
tangent  to  each  other,  the  internal  surface  would  be  about  fifteen  times  th< 
the  corresponding  outer  faces  of  the  leaf.  This,  of  course,  does  not  pretend  to  pii  - 
ture  the  actual  state  of  affairs;  but  it  will  give  an  idea  of  the  relative  magnitudes 
involved. 

Stomata.  — The  guard  cells  of  the  stomata  are  different  from  the  rest 
of  the  epidermal  cells  in  form,  in  the  peculiar  unequal  thickening  of 
their  walls,  and  generally  in  the  possession  of  chloroplasts.  These 
characters  and  the  position  of  the  guard  cells  with  reference  to  the  ad- 
jacent subsidiary  cells  determine  simultaneous  differences  in  turgor 
and  make  them  behave  differently  from  the  others.  In  general  when 
turgid,  they  become  arcuate,  and  when  flaccid,  they  straighten.  The 
mechanics  of  these  movements  differs  considerably  with  differences  of 
form,  structure,  and  position,  and  in  none  of  the  several  types  that  have 
been  distinguished  is  it  fully  understood.  The  chloroplasts  are  supposed, 
but  on  no  very  good  grounds,  to  impart  power  to  make  osmotically 
active  substances  that  do  not  exist  in  adjacent  cells  (or  are  presenl  in 
smaller  amount),  so  that  these  cells  may  be  more  turgid  than  the  others 
with  the  same  water  supply.  The  longitudinal  thickenings  are  elasti 
and  are  supposed  to  straighten  the  cells  when  they  become  flaccid. 
The  auxiliary  cells  are  supposed  to  offer  proper  bracing  for  the  guard 
cells  so  that  turgor  will  arch  them. 

Regulation  by  stomata.  —  Naturally  the  guard  cells  are  most  likely 
to  be  turgid  when  the  water  supply  is  good;   then  the  opening  of  the  slit 


328  PHYSIOLOGY 

between  them  permits  free  diffusion  of  the  water  vapor  into  the  outer 
air.  Conversely,  the  guard  cells  become  flaccid  with  scant  water, 
straighten  elasticaily,  and  practically  close  the  slit.  This  sort  of  adjust- 
ment is  held  to  "  regulate  "  the  transpiration,  permitting  it  when  water 
is  abundant,  reducing  it  when  the  supply  is  inadequate.  Yet  if  the 
assumption  of  the  existence  of  special  osmotically  active  substances  in 
the  guard  cells  were  correct,  they  should  be  the  last  to  feel  the  slackening 
of  the  water  supply;  and  so  one  must  assume  further  that  they  are  ad-  ' 
justed  to  water  much  as  are  the  other  green  cells  of  the  leaf  —  an  assump- 
tion which  is  hardly  justifiable  in  view  of  their  position  and  connections. 
Of  course  the  immediate  effect  of  the  reduction  in  area  of  the  stomatal 
slits  is  to  reduce  the  amount  of  vapor  diffusing  through  them.  But 
this  in  turn  would  increase  the  relative  humidity  of  the  internal  atmos- 
phere (i.e.  that  of  the  intercellular  spaces),  would  cause  the  accumula- 
tion of  a  "  head  "  of  pressure,  so  to  speak,  that  would  accelerate  the  dif- 
fusion through  the  narrower  slit,  and  the  system  would  tend  to  reach  an 
equilibrium  again.  Thus  the  closure  of  the  stomata  is  rendered  par- 
tially or  wholly  ineffective.  Were  the  internal  atmosphere  saturated 
with  moisture  when  stomata  are  open  (as  has  been  assumed) ,  the  closure 
could  not  have  this  effect.  But  this  assumption  has  not  proved  correct. 
Other  changes  in  the  external  world  (i.e.  stimuli)  affect  the  guard  cells. 
Of  these  light  is  the  most  notable.  In  general  the  guard  cells  curve  in 
light  and  straighten  in  darkness;  the  tendency,  then,  is  for  the  stomata 
to  open  at  a  time  when  the  evaporation  is  greatest  and  to  close  when  it 
is  least. 

It  is  difficult  to  reconcile  the  facts  with  the  commonly  accepted  view 
that  the  stomata  are  "  delicately  balanced  valves  "  which  adjust  trans- 
piration to  the  "  needs  "  of  the  plant. 

If  the  logic  on  which  that  idea  rests  were  valid,  it  would  prove  rather 
that  the  stomata  regulate  the  admission  of  gases,  since  any  diminution 
of  the  size  of  the  slit  must  diminish  the  amount  of  C02  admitted  to  the 
air  passages,  no  "  backing  up  "  and  accumulation  of  a  "  head  "  of  pres- 
sure being  possible  in  this  case,  whereas  it  does  occur  with  water  vapor 
diffusing  from  the  plant.  In  the  absence  of  any  apparent  advantage 
in  regulating  the  movement  of  gases,  and  the  "  need  "  of  some  control 
over  evaporation,  it  has  been  assumed  that  the  stomata  are  able  to 
adjust  matters  so  that  enough  water  will  flow  through  the  plant,  carry- 
ing with  it  needed  salts,  while  at  any  time  these  governors  can  check 
loss  when  danger  threatens.    Many  cases,  however,  have  been  reported 


THE    M  \  I  I : l< I  \l.   i  »l K',(  »   <  »i     PLANTS 


329 


in  which  the  guard  cells  are  immobile,  or  respond  very  sluggishly  to 
external  stimuli.  Further,  the  more  exael  become  the  studies  on  plants 
of  desert  regions,  where  the  need  of  an  effective  regulating  mechan- 
ism seems  most  obvious,  the  less  efficient  do  the  stomata  appear. 
Rather  it  appears  that  they  are  scarcely  more  than  a  tardily  acting  mech- 
anism which  may  save  the  plant  in  extremity,  hut  does  not  produce  any 
exacl  adjustment. 

Factors  in  transpiration. — The  amount  of  water  losl  from  a  given 
surface  of  plant  tissue  is  extremely  variable.  The  humidity  of  the  air, 
its  temperature  and  pressure,  which  also  affect  humidity,  and  the  tem- 
perature of  the  plant  are  the  chief  factors  which  cause  the  rate  of  evapora- 
tion to  vary.  The  simplest  mode  of  determining  evaporation  quanti- 
tatively is  by  weighing  potted  plants  at  intervals,  having  prevented 
evaporation  from  the  surface  of  pot  and  soil  by  some  impervious  cover- 
ing of  rubber,  metal,  or  wax.  It  is  not  justifiable,  however,  to  apply 
these  data  to  plants  in  nature. 

Humidity.  —  In  a  saturated  atmosphere  there  can  be  no  water  loss. 
Yet  experimentally  this  is  very  difficult  to  establish.  The  reason  is  to 
be  sought  in  two  directions.  First,  it  has  been  found  practically  im- 
possible to  maintain  an  atmosphere  absolutely  saturated  at  all  times, 
for  that  means  an  invariable  temperature,  which,  under  other  conditions 
necessary  to  the  experiment,  is  unattainable.  Second,  even  were  the 
proper  external  conditions  attained,  the  plant  by  respiration  would  be 
a  little  warmer  than  the  air,  and  the  air  next  the  plant,  therefore,  would 
not  be  quite  saturated;  so  some  small  amount  of  evaporation  might  take 
place.  Yet  during  rain,  mist,  or  fog,  practically  no  evaporation  occurs; 
and  as  the  humidity  decreases  from  ioo  per  cent  to  the  70  per  cent  of  a 
moderate  day  or  to  the  50  per  cent  of  a  dry  day,  evaporation  increases. 
As  the  humidity  fluctuates  from  day  to  day  or  even  from  hour  to  hour, 
the  evaporation  varies  likewise.  The  most  marked  changes  in  relative 
humidity  are  due  to  the  rising  or  falling  temperature  of  the  air.  As 
temperature  rise-,  relative  humidity  becomes  less,  the  heat  energy  im- 
parted to  the  plant  is  greater,  arid  evaporation  is  increased  by  both 
causes. 

Barometric  pressure.  —  As  the  air  pressure  is  reduced  the  boiling  point 
of  water  falls;  so  fluctuations  in  the  barometer  indi.  ate  inverse  changes 
in  the  rate  of  transpiration.  Yet  these  variation-  at  any  locality  are 
insignificant;  the  reduction  in  air  pressure  becomes  important  only  in 
comparing  plants  at  high  and  low  altitudes.     In  alpme  regions,  when 


330  PHYSIOLOGY 

low  barometer  may  coincide  with  low  humidity  and  therefore  intense 
light,  the  excessive  evaporation  often  becomes  a  powerful  factor  in 
dwarfing  plants  and  in  controlling  their  distribution. 

Temperature. — The  temperature  of  the  plant  itself  tends  normally 
to  equal  that  of  the  air,  since  its  extended  surface  permits  quick  gain 
or  loss  of  heat  toward  equilibrium.  A  rise  of  temperature  in  the  air, 
therefore,  is  quickly  followed  by  a  rise  of  temperature  in  the  plant,  and 
(even  with  no  change  in  the  relative  humidity  of  the  air)  by  increased 
evaporation.  But  the  temperature  of  the  plant  depends  also  upon  the 
energy  absorbed  by  the  green  pigment  in  diffuse  light  or  direct  sunlight. 
In  diffuse  light  the  greater  part  of  this  energy  is  used  in  food  making, 
and  only  a  small  portion  exerts  a  heating  effect.  But  in  sunlight  two 
thirds  to  three  fourths  of  that  absorbed  is  free  to  heat  the  tissues,  and  as 
soon  as  that  begins,  evaporation  is  thereby  much  accelerated.  This 
tends  to  dissipate  the  heat. 

It  has  been  proposed  to  call  the  evaporation  due  to  the  excess  of  energy  absorbed 
by  the  chlorophyll,  chlorovaporization.  The  term  has  its  only  value  in  promoting 
recognition  of  the  fact;  but  chlorovaporization  cannot  be  distinguished  practically 
from  the  rest. 

Were  it  not  for  this  transfer  of  energy  to  the  water  vapor,  the  tempera- 
ture of  the  tissues  would  rise  to  the  danger  point,  or  at  least  to  a  degree 
which  retards  food  making.  When  transpiration  is  greatly  reduced  by 
enclosing  a  shoot  in  a  glass  chamber  whose  air  quickly  becomes  nearly 
saturated  while  the  light  is  absorbed,  death  quickly  ensues.  The 
"  scalding  "  of  leaves  by  sunshine  after  a  summer  shower  is  an  example 
of  the  same  effect.  If  a  plant  derives  no  other  advantage  from  tran- 
spiration, this  prevention  of  injury  by  overheating  in  direct  sunlight 
is  certainly  one.  For  even  temporary  interference  with  food  making 
might  be  serious,  and  permanent  stoppage  of  it  by  the  killing  of  any 
considerable  area  of  leaves  might  be  fatal  to  the  whole  plant.  How- 
ever possible  it  might  be  for  plants  to  meet  this  difficulty  by  other 
methods,  if  transpiration  could  be  eliminated  for  other  reasons,  under 
the  present  organization  transpiration  is  of  real  advantage  in  this 
particular. 

Amount  transpired.  —  Because  of  the  extreme  variation,  from  zero 
to  the  maximum,  a  quantitative  statement  of  the  amount  of  evaporation 
is  of  little  value,  though  a  voluminous  literature  records  an  enormous 
number  of  observations  and  calculations.  The  following  will  serve  as 
illustrative  examples. 


THE   MATERIAL   OUTGO   OF    PLANTS  331 

Measured  evaporation  from  100  sq.  cm.  of  leaves  (200  sq.  cm.  of  surface) 
in    bright    hii  fuse    light,   at   about    20°    c,    with     humidity   about 

50   PER   CENT 

I  hr.  24  hr. 

Phaseolus  vulgaris 0.117  gm.  2.81  gm. 

Hedera  Helix       0.17  4.09 

Begonia  argentea 0.19  4.57 

Colcus  Blumei 0.21 1  5.06 

Cucurbita  Pcpo 0.224  5-39 

Ficus  elastira        0.262  6.3 

Helianthus  annuus 0.5  12.0 

Lupinus  albus 0.594  14.27 

Chrysanthemum  frutescens 0.681  16.35 

Vicia  Faba 0.683  16.4 

Hemp  plants  in  a  season  of  140  days  were  estimated  to  evaporate  (each)  27  kg. 
and  sunflowers  66  kg.  of  water.  It  is  estimated  that  if  the  water  evaporated  by  the 
following  cereals  were  again  condensed  on  the  area  occupied  by  each  sort,  say  1 
sq.  m.,  it  would  cover  the  ground  in  the  case  of  rye  to  a  depth  of  83  mm., 
wheat,  11S  mm.,  and  oats,  127  mm.  The  average  annual  rainfall  in  the  north  <  en- 
tral  states  is  in  the  neighborhood  of  1000  mm.,  so  that  one  twelfth  to  one  eighth  of 
the  total  passes  through  such  cereals. 

A  birch  tree  with  200,000  leaves  is  estimated  to  evaporate  on  a  hot  day  300  to 
400  kg.  A  beech,  15  years  old,  is  said  to  average  about  75  kg.  per  day  in  the  months 
from  June  to  September,  inclusive.  At  thai  rale  a  he<  tare  of  bee<  h  forest  contain- 
ing 400-600  trees  would  evaporate  some  20,000  barrels.  In  all  these  calculations 
and  estimates  a  liberal  allowance  must  be  made  for  errors. 

Reduction  of  water  loss.  —  Among  all  the  agencies  that  affect  the  form 
and  mode  of  development  of  plants  none  has  more  influence  than  water, 
and  the  relation  between  the  available  supply  and  the  loss  by  evapora- 
tion. In  the  peculiarities  of  form  and  structure  which  seem  related 
particularly  to  water,  many  see  "  adaptations  "  to  a  habitat  with  much 
water,  a  moderate  amount,  or  a  scanty  supply.  Thus  the  cutinization 
of  the  epidermis,  the  formation  of  a  waxy  or  resinous  coating,  and  the 
development  of  cork  are  structures  which  reduce  the  loss  of  water. 
In  other  plants  the  scanty  or  fleshy  foliage,  the  complete  absence  of 
leaves,  the  development  of  water-holding  tissues,  the  short  cylindric  or 
globular  fleshy  body,  the  deep-running  roots,  and  many  other  peculi- 
arities (treated  more  fully  in  Part  III,  Ecology)  are  considered  as 
"  adaptations  "  to  a  dry  climate.  It  would  be  better  to  look  upon 
them  as  effects  of  climate  and  similar  factors,  since  experiments  indicate 
that  such  "  adaptations"  can  be  produced  at  will,  even  in  one  generation, 
by  cultivation  under  appropriate  conditions. 


332  PHYSIOLOGY 


2.    EXUDATION    OF    WATER 

Forms  of  exudation.  —  Besides  the  vapor  which  constantly  exhales 
from  plants,  liquid  water  exudes  from  certain  regions  intermittently. 
The  places  whence  it  issues  are,  first,  certain  specially  permeable  areas 
of  the  permeable  regions  in  an  uninjured  plant;  second,  the  conducting 
tissue  when  opened  by  some  wound.  Guttation  is  the  escape  of  water 
in  drops  from  uninjured  plants.  It  occurs  especially  in  leaves  in  the 
vicinity  of  the  tips  of  main  veins,  where  there  are  stomata,  often  enlarged, 
called  water  pores,  through  which  water  exudes.  Bleeding  is  the  oozing 
of  water  from  the  water-conducting  tissues  when  ruptured.  It  is  espe- 
cially notable  in  the  spring,  before  the  foliage  is  fully  developed.  Secre- 
tion consists  in  the  exudation  of  water  and  solutes  from  certain  special- 
ized cells,  constituting  a  gland,  and  found  on  various  parts  of  plants,  but 
especially  on  foliage  and  flower  leaves.  All  these  processes  are  essen- 
tially similar,  with  minor  differences. 

Guttation.  — Guttation  may  be  readily  observed  by  inverting  a  glass 
jar  over  grass  seedlings  growing  in  well-watered  soil  and  thus  checking 
the  evaporation.  In  a  short  time  a  water  drop  accumulates  at  the  tip 
of  the  blade  and  enlarges  until  it  runs  down  or  falls  off.  Leaves  of  vigor- 
ous plants  of  many  species  {e.g.  aroids,  fuchsia,  cabbage,  nasturtium) 
under  like  conditions  show  droplets  of  water  at  the  tips,  or  at  marginal 
teeth,  or  near  the  end  of  main  ribs. 

Accessory  structures.  —  In  all  these  cases  an  examination  shows  es- 
sentially the  same  features:  (a)  a  rift  in  the  epidermis,  or  one  or  more 
water  pores,  over  (b)  a  rather  large  chamber,  which  is  bounded  by  (r) 
more  or  less  specialized  colorless  parenchyma  cells  (epithem),  and  near 
by  (d)  the  tracheids  at  the  end  of  a  vein.  The  rift  in  the  epidermis  may 
be  due  (as  in  grasses)  to  growth  and  consequent  stretching  and  rupture. 
The  water  pore  is  simply  a  deformed  stomatal  apparatus  whose  dilated  slit 
is  always  wide  open  because  the  distorted  guard  cells  are  no  longer  motile. 
When  the  water  pore  is  single,  it  is  usually  greatly  enlarged  and  deformed ; 
when  there  are  a  number  together,  each  is  more  nearly  like  an  ordinary 
wide-open  stoma.  The  cells  lining  the  substomatal  chamber  differ  from 
the  mesophyll  cells  chiefly  in  lacking  chloroplasts.  They  resemble  the 
sheath  of  colorless  cells,  the  so-called  transfusion  tissue,  that  adjoins  the 
tracheids,  which  form  the  endings  of  the  water-conducting  bundles  of 
the  leaves.     In  some  cases  this  epithem  seems  to  be  a  water-secreting 


THE    MATERIAL   OUTGO   OF    PLANTS 


333 


tissue  and  to  deserve  the  name  water  gland;    in  others  it   seems  to  be 
passive. 

Guttation  in  fungi. — Guttation  is  ool  confined  to  the  higher  plant-, 
nor  are  there  always  sueh  elaborate  accessory  structures.  It  occurs  in 
its  simplest  form  in  many  fungi.  Thus  Pilobolus  crystallinus  owes  its 
specific  name  to  the  droplets  of  water  which  appear  on  its  sporangio- 
phores  (fig.  630),  and  Mendius  lacrymans,  the  dry  rut 
fungus,  likewise,  "  weeps  "  so  much  water  that  it  accum- 
ulates in  big  drops  on  the  surface  of  its  sheetlike 
mycelium. 

Nightly  guttation.  —  In  nature  the  checking  of  evapo- 
ration, which  results  in  guttation,  occurs  chiefly  at  night, 
when  many  young  plants  exude  water.  What  remains 
adherent  at  the  water  pore  may  be  partly  resorbed  when 
transpiration  begins. 


This  seems  to  be  the  way  in  which  a  destructive  bacterial  dis- 
ease of  cabbage  infects  the  plants.  By  contamination  of  the 
hanging  drop  the  bacteria  find  their  way  into  the  chamber  as  the 
drop  evaporates  or  is  resorbed,  there  develop  and  so  kill  the 
adjacent  cells,  whence  they  enter  the  xylcm  bundles  and  work 
backward,  killing  and  rotting  the  bundles.  When  the  crop  is 
gathered  and  stored,  they  develop  further,  until  the  head  is  spoiled 
by  the  extension  of  the  blackened  and  rotted  tracts  in  the  blanched 
leaves. 


Fig.  630.  — 
Sporangiophore 
of  Pilobolus, 
showing  ex- 
uded water. — 
Adapted     from 

ZOPF. 


One  may  easily  observe  the  exudation  of  water  from  the  leaves  of 
lawn  grasses  early  in  the  evening,  when  the  "  dew  "  is  said  to  be  "  fall- 
ing." The  warm  soil  conduces  to  the  entry  of  water;  the  cooler  air 
checks  evaporation;  these  conditions  permit  maximum  turgor;  gutta- 
tion at  the  tips  of  uninjured  Leaves,  or,  more  often  and  more  promptly, 
bleeding  from  the  cut  ends  of  the  leaves  is  the  result.  Dew,  of  course, 
may  form  under  proper  conditions;  but  exuded  water  forms  a  great  part 
of  what  passes  as  dew. 

Artificial  guttation.  — Guttation  may  be  produced  artificially  by  injecting  water 
under  pressure  into  the  stem  of  a  plant  known  to  have  water  pores,  as  by  attaching 
the  end  of  a  cut  shoot  to  a  water  tap.  Presently  droplets  exude  at  the  usual  places. 
It  is  usually  assumed  that  the  water  is  thereby  forced  through  the  plant  tissues, 

but  as  city  water  pressure  varies  from  2—3  atmospheres  (seldom  more,  and  less 
will  often  answer),  it  is  doubtful  if  so  low  a  pressure  ias  compared  with  the  3—10 
atmospheres  of  common  turgor  pressure)  would  be  adequate  to  do  this  (see  further, 
P-  3&)- 


334  PHYSIOLOGY 

Quantity  exuded.  —  In  a  few  plants,  especially  in  aroids,  guttation  under  favorable 
conditions  is  so  rapid  that  water  drips  from  leaf  tips  or  is  even  ejected.  Thus  a 
vigorous  leaf  of  Colocasia  has  yielded  1008  cc.  of  water  in  9  days,  the  water  drop- 
ping at  the  rate  of  85-100  drops  per  minute  at  times.  C.  nymphaeoides  has  been 
observed  to  eject  a  stream  of  minute  droplets  (at  a  rate  of  195  per  minute,  so  that  it 
seemed  almost  a  continuous  jet  of  water)  to  a  height  of  about  1  cm. 

Advantage?  —  Seeing  the  structural  features  which  permit  guttation, 
one  naturally  asks,  Is  it  advantageous?  To  that  question  no  certain 
answer  can  be  given.  It  is  assumed  that  the  free  escape  of  water  at 
these  points  prevents  its  escape  elsewhere,  and  therefore  prevents  the 
infiltration  of  the  aerating  system  with  water,  which  would  greatly  retard 
the  entry  of  gases  and  so  the  manufacture  of  food.  But  there  are  so 
many  plants  which  lack  the  arrangements  for  guttation  that  one  must 
doubt  if  this  answer  be  adequate. 

Bleeding.  —  Bleeding  may  be  observed  when  vines  are  pruned  rather 
late,  or  in  many  cases  when  a  potted  plant  is  decapitated.  It  must  be 
distinguished  from  exudation  due  to  heating  the  water  and  especially 
the  gases  contained  in  the  woody  parts  of  a  plant,  which  has  the  same 
general  effect.  Thus,  when  a  green  stick  is  put  on  the  fire,  the  scanty  sap 
presently  boils  out  of  the  ends;  for  the  expansion  of  the  gases  and  of 
the  water,  and  later  the  steam  generated  by  the  fire,  drive  it  out  forcibly. 
Or  if  on  a  cold  day  in  winter,  one  bring  into  a  warm  room  a  branch  of 
a  shrub  or  tree,  water  will  soon  ooze  out  at  the  cut  surface.  Here  the 
gases  in  the  wood  are  warmed  (for  though  fuller  of  water  in  winter  than 
at  other  times,  the  wood  is  never  free  from  gases,  else  no  green  wood 
would  float);  they  expand,  and  press  upon  the  free  water,  forcing  it  out 
at  the  nearest  opening.  True  bleeding,  however,  is  restricted  to  live 
plants  and  is  quite  independent  of  any  gas  pressure  due  to  heat. 

Industrial  applications. — Collecting  maple  sap  for  sugar  or  sirup  making  is 
partly  an  industrial  application  of  bleeding.  The  work  is  often  begun  when  only 
the  heating  of  the  twigs  on  a  warm  sunny  day  is  active  in  forcing  out  the  water 
through  the  wound  made  in  the  trunk;  but  a  great  part  of  the  later  exudation  is 
dependent  on  other  causes  and  must  be  accounted  to  this  extent  as  true  bleeding. 
Another  commercial  application  of  bleeding  is  found  in  the  collection  of  the  sap 
of  various  species  of  Agave  in  Mexico  and  Central  America  for  the  manufacture  of 
fermented  and  distilled  liquors.  The  process  begins  with  cutting  out  the  huge  bud 
at  the  time  when  the  plant,  at  the  end  of  5-15  years'  growth,  is  about  to  send  up  the 
great  flower  stalk,  12-20  cm.  in  diameter  and  6-10  m.  high.  Into  the  basin  formed 
by  removing  the  bud,  the  plant  exudes  several  liters  of  water  a  day,  for  two  months 
or  more;  this  is  collected  daily,  and  after  the  addition  of  milk  and  fermentation  is 
esteemed  as  a  beverage,  called  pulque.     Extensive  plantations  are  devrted  to  rais- 


THE   MATERIAL  OUTGO   OF   PLANTS  335 

ing  the  agave  or  maguey,  and  pulque  trains  run  into  lli«'  large  I  ilics,  as  milk  trains 
do  in  tin's  country.  The  fermented  sap  is  also  distilled  to  make  various  fiery  alco- 
holic drinks. 

Conditions. — The  conditions  under  which  bleeding  occurs  are  like 
those  for  guttation,  a  liberal  water  supply  and  limited  transpiration; 
that  is,  the  conditions  which  permit  maximum  turgor.  Even  so,  not  all 
plants  bleed;  hence  it  cannot  be  at  all  necessary,  nor  can  the  causes  be 
universally  active. 

Cause  of  exudation. — The  cause  of  bleeding  and  guttation  is  to  be 
sought  in  the  development  of  high  turgor  in  certain  cells  (on  account  of 
the  osmotic  pressure  of  the  solutes  in  them  to  which  the  protoplast  is 
impermeable),  which  is  made  possible  by  adequate  water  supply.  To 
stop  evaporation  by  making  the  air  about  the  aerial  parts  very  moist,  or 
by  cutting  away  the  aerial  parts,  or  to  have  limited  evaporation  because 
the  foliage  is  not  yet  fully  developed,  are  merely  ways  by  which  a  water 
supply,  that  might  otherwise  be  barely  enough  to  cover  the  evaporation, 
is  made  ample;  and  this  permits  high  turgor  when  other  conditions  are- 
met.  When  the  turgor  rises  to  a  certain  point  in  the  active  cells,  it  seems 
that  water  is  exuded. 

This  may  be  mere  filtration  under  pressure.  But  we  may  also  conceive  it  to  be 
due  to  a  sudden  alteration  of  the  permeability  of  the  cytoplasm,  wrought  by  the 
very  pressure  itself.  In  that  event,  upon  the  relief  of  pressure  when  the  outgo  oc- 
curs, there  would  be  a  gradual  recovery  of  impermeability  and  consequently  of 
turgor  to  the  maximum;  then  another  automatic  change  of  permeability,  a  conse- 
quent outrush  of  water,  and  so  on. 

This  outflow  naturally  cannot  be  pure  water  :  but  on  the  theory  of 
filtration  the  water  will  contain  at  least  the  substances  to  which  the  pro- 
toplast is  permeable;  and  on  the  second  hypothesis,  any  or  all  solutes 
might  be  released,  the  sap  as  a  whole  escaping.  In  the  water  there  are 
often  substances  in  small  amount,  regarding  whose  osmetic  relations  we 
are  ignorant,  though  the  general  assumption  is  that  they  could  not  pan- 
tile cytoplasm  without  some  special  modification  of  its  permeability. 
When  that  is  demonstrated,  it  will  be  necessary  to  adopt  the  second 
hypothesis,  which  is  also  used  to  account  for  the  presence  of  such  ^lb- 
stances  in  secretions  (see  p.  340).  Until  then  it  will  suffice  to  assume 
that  they  issue  with  the  water  because  they  are  free  to  do  so. 

Tissues  concerned.  —  In  the  case  of  Pilobolus  and  like  plans,  the  tur- 
gor which  causes  the  escape  of  water  evidently  arises  in  the  verv  cell  or 


336  PHYSIOLOGY 

coenocyte  from  which  it  escapes.  This  may  also  be  the  case  in  guttation 
among  seed  plants.  The  epithem  of  the  water  chamber,  receiving  an 
adequate  supply  of  water  from  the  adjacent  vein,  may  develop  turgor 
sufficient  to  cause  water  to  pass  the  cytoplasm  and  the  wall.  It  is  ob- 
vious that  to  issue  from  the  free  surface  it  will  encounter  less  resistance 
than  elsewhere;  consequently  it  takes  this  direction.  The  chamber 
fills  and  water  soon  oozes  from  the  water  pore.  But  the  epithem  can- 
not develop  an  adequate  turgor  unless  the  water  supply  is  sufficient. 
That  may  be  made  sufficient  either  by  checking  the  transpiration,  or  by 
forcing  water  up  to  these  cells  so  that  they  may  get  enough,  even  though 
transpiration  is  unchanged.  Water  may  be  supplied  thus  artificially  by 
cutting  the  stem  and  attaching  it  to  a  water  tap;  or  the  same  end  would 
be  accomplished  in  nature  if  the  root  cortex  had  a  supply  adequate  to 
enable  it  to  become  fully  turgid  and  exude  water  under  pressure  into  the 
conducting  system. 

"  Root  pressure."  — The  condition  just  mentioned  often  exists  in  the 
root  cortex,  and  perhaps  always  when  plants  are  not  flaccid.  The  loca- 
tion of  this  turgor  has  suggested  for  it  the  name  "  root  pressure."  This 
is  unfortunate,  because  it  tends  to  obscure  the  fact  that  any  live  thin- 
walled  cells  with  like  conditions  may  develop  a  turgor  which  will  cause 
water  to  exude.  Thus,  bleeding  was  found  to  occur  in  the  inflorescence 
of  some  tall  palms,  but  the  root  cortex  had  no  part  in  so  distant  an  exuda- 
tion; the  pressure  originated  near  the  base  of  the  flower  stalk.  The 
"  root  pressure  "  being  a  frequent  cause  of  bleeding,  the  phrase  "  bleed- 
ing pressure  "  has  been  suggested  as  a  substitute;  but  this  is  little  better, 
since  whether  or  not  bleeding  results  is  purely  incidental.  No  special 
term  is  needed  other  than  turgor  pressure;  that  is  general  and  is  specific 
enough.     (See  also  p.  349.) 

Amount  and  pressure.  —  Experiments  on  bleeding  are  often  con- 
ducted with  potted  plants,  which  are  decapitated,  and  to  the  stump  is 
affixed  apparatus  for  measuring  the  amount  of  water  exuded,  or  the 
pressure  with  which  it  is  forced  out.  With  trees,  the  trunk  is  bored 
and  the  receptacles  or  gages  attached.  A  few  examples  will  give  an 
idea  of  the  maximum  quantity  of  the  sap  and  the  pressures  involved. 

A  calla  lily  bled  39  cc.  in  24  hours.  A  vigorous  European  grape  some- 
times exudes  nearly  a  liter  per  day.  The  Mexican  agaves,  cultivated 
for  this  purpose,  are  said  to  give  out  5-6  liters  daily  for  several  months. 
Under  favorable  conditions,  the  sugar  maple  yields  5-8  liters  in  the 
course  of  a  day,  and  the  birches  give  out  about  as  much. 


THE   MATERIAL   OUTGO   OF   PLANTS 


637 


The  pressures,  recorded  in  millimeters  of  mercury  (700  =  1  atmosphen 
from  o  to 

Ribcs  rubrum  (red  currant) 358 

Acer  platanoides  (sycamore  maple)       ,347 

Acer  saccharum  (sugar  maple) 1033 

Psedera  quinquefolia  (Virginia  creeper) 615 

Betula  alba  (white  birch) 1390 

Betula  lutea  (yellow  birch)        1.S15 

Betula  lenta  (black  birch) 2040 

Vitis  vinifera  (European  grape) 860 

Much  study  has  been  given  to  variations  in  the  amount  and  pressure 
of  bleeding;  seasonal  and  possibly  diurnal  fluctuations  have  been  dis- 
covered; but  inasmuch  as  turgor  pressure  must  be  influenced  by  tran- 
spiration, itself  of  infinite  variability,  the  precise  results  of  these  studies 
are  not  important. 

The  limited  movement  of  water  through  submersed  aquatics  which 
has  been  described  cannot  be  due  to  transpiration,  and  is  probably  not 
a  case  of  guttation.  The  experimental  evidence  is  scanty  and  the 
movement  may  be  referable  to  the  larger  heating  effects  on  the  Leaves 
as  compared  with  the  stems.  This  should  create  a  slow  movement  of 
water  out  of  the  leaf,  to  be  supplied  from  below. 

Secretion.  —  Secretion  is  a  much  more  general  and  varied  phenomem  >n 

than   guttation   or   bleeding.     It   is  performed   by   more   limited   and 

specialized  tissues,  called  glands,  and  the  variety  of  substances  which 

escape  is  much  greater,  though  the  amounts  lost  are  much  smaller. 

Many  of  the  secretions  are  of  such  a  nature  that  they  play  an  important 

part  in  the  life  of  the  plant;  others  are  of  no  use  so  far  as 

we  know  and  are  therefore  called  waste  products.    No  dis-        (l£^y 

tinction  can  be  made  in  plants  between  useful  secretions  |g| 

and  waste  excretions.  £**• 

Glands.  — There  are  some  glands  which  secrete  water,     „  F10-6^-  — 

,.    .  ,  ,.,  ,  .  ,  .        ^oung  gland- 

with    no   distinctive   solutes,    like   that   which   escapes   in     uiar    hair    oi 

guttation   and  bleeding ;    and  because   there  are  no  dis-     Pdarpmium  : 

tinctive   solutes   these   are  called   water  glands.     Glands    volatile  oil.— 

are  named  usually  according  to  the  most   abundant   or     lrom    1>;Ur 

characteristic  material  they  -circle.    Thus  those  in  whose 

secretion  calcium  salts  become  conspicuous  by  concentration  are  called 

lime  glands;   digestive  glands  secrete  water  containing  enzymes.    Most 

common  of  all  are  the  nectar  glands  or  nectaries,  abundant  in  flowers, 

but  found  also  on  oilur  parts  as  extra-floral  nectaries  (fig.  1183),  whose 


338 


PHYSIOLOGY 


water  is  sweet  with  sugar  and  often  fragrant.  Not  all  glands,  however, 
secrete  water  and  its  solutes.  There  are  glands  whose  secretion  is  an 
essential  oil,1  of  which  a  great  variety  are  formed.  Still  others  secrete 
resin,  which  may  be  formed  from  an  essential  oil. 

Form  of  glands.  — The  form  of  glands  is  various.  A  single  epidermal 
cell  may  differ  from  its  neighbors;  it  may  be  level  with  them,  or  sunk,  or 
raised  upon  a  shorter  or  longer  stalk,  like  the  glandular  hairs  (fig.  631). 
A  filament  or  a  cluster  of  such  cells  may  form  a  stalked  gland  (fig.  632); 

the  gland  cells 
may  form  a 
rather  indefinite 
mass,  or  they 
may  line  a  shal- 
low cavity  (fig. 
633),  or  a  deep 
pouch,  as  in  the 
nectary  of  the 
nasturtium  (fig. 
634);  or  they 
may  be  the  epi- 
thelium of  a  sim- 
ple or  branched 
duct,    as    in   the 

lilies  (fig.  635).  Nor  do  all  glands  pour  out  their  secretion  on  the 
surface.  The  gland  cells  may  part  when  young,  forming  intercel- 
lular spaces  into  which  the  secretion  exudes  to  escape  through  water 
pores.  Or  a  single  intercellular  space  may  develop  in  the  center  of  the 
group  (fig.  636)  which  receives  the  secretion;  then  as  the  gland  and  space 
grow,  the  secreting  cells  form  an  epithelium  for  a  closed  reservoir, 
larger  or  smaller,  containing  the  secretion.  Or,  later,  by  the  destruction 
of  the  gland  cells  loaded  with  the  secretion,  it  finally  occupies  their  place 
as  well  as  the  intercellular  space,  and  reaches  the  surface  only  by 
mechanical  rupture  of  enveloping  tissues  (fig.  637). 

Emission  of  secretions.  —  Very  little  is  known  of  the  chemical  pro- 
cesses by  which  the  peculiar  materials  of  the  secretion  are  formed. 
Each  sort  of  gland  doubtless  pursues  a  different  course.  Nor  is  it  pos- 
sible to  account  for  the  emission  of  the  various  substances.     Some,  like 

1  Not  true  oils,  from  which  they  may  be  distinguished  by  making  only  a  transient  grease 
spot  on  paper. 


Fig.  632.  —  Gland  (g)  from  the  upper  surface  of  the  leaf  of 
lilac  (Syringa  vulgaris):  e,  epidermis;  c,  cuticle;  p,  p,  palisade 
cells  ;  i,  intercellular  space. 


THE    MATERIAL    OUTGO   OF    PLANTS 


339 


cane  sugar,  are  known  to  be  re- 
tained ordinarily  by  the  cyto- 
plasm ;  yet  nectar  glands  se<  rete 

sugar  one  or  more  times.  Others, 
for  example  enzymes,  have  a  com- 
position which,  though  imper- 
fectly known,  is  such  as  to  suggest 
that  the  cytoplasm  would  usually 
be  impermeable  to  them;  yet  di- 
gestion occurs  in  such  places  as  to 
make  it  certain  that  enzymes  are 
able  to  pass  out  of  the  cells  in 
which  they  arise. 


Fig.  633.  —  Section  through  a 
petal  of  buttercup  (Ranunculus), 
showing  nectar  gland  (n)  and 
shallow  receptacle  formed  by 
the  "nectary"  (a).  Note  bundle 
of  conducting  tissues  (x).  — 
After  Bonnier. 


Fig.  634.  —  Flower  of  nastur- 
tium (Tropin rolum  mains)  cut 
through  the  middle  to  show  the 
spur  (5)  and  the  nectar  («). 


Fig.  635. —  Net  tar  gland  in  the 
ovary  of  day  lily  {HemerocaUis 
Jiava).  —  After    ScHNIl  WIND- Tim  S. 


34o 


PHYSIOLOGY 


The  problem,  therefore,  is:  How  can  solutes  pass  the  ectoplast  usually 
impermeable  to  them  ?  The  answer  is  merely  in  the  form  of  a  hypothe- 
sis, like  the  one  already  proposed  to  account  for 
guttation  and  bleeding.  If  the  accumulation  of 
the  solute  causes  a  rise  of  turgor,  it  is  conceivable 
that  the  very  pressure  itself  might  work  such  a 
change  in  the  cytoplasmic  membranes  that  they 
alter  their  permeability  and  permit  the  outrush 
of  water  and  its  solutes  in  the  direction  of  least 
resistance,  which  will  be  toward  the  free  surface. 
Whether  a  renewed  secretion  will  take  place  de- 
pends on  the  further  activity  of  the  cell.  Given 
a  repeated  formation  of  the  secretion,  it  might 
escape  again.  The  hypothesis  then  suggests  a 
rhythmic  variation  in  the  permeability  of  the  cell 
membranes,  the  secretion  being  formed  inside  the  cell. 


Fig.  636.  —  Young 
resin  gland  of  fir  (Abies): 
a,  duct,  an  intercellular 
space  formed  by  the  sepa- 
ration of  the  four  nucleate 
cells.  —  After  Tschirch. 


This  hypothesis  is  clearly  inapplicable  to  secretions  which  are  not  miscible  with 
water,  like  essential  oils  and  resins.  They  are  probably  formed,  however,  in  the  very 
wall  itself,  and  thus  the  material 
may  not  have  to  traverse  the  ecto- 
plast as  resin  or  oil.  Unfortunately, 
even  the  place  of  their  origin  is  still 
obscure. 

Role  of  certain  secretions.  — Nec- 
tar is  gathered  by  many  insects, 
some  of  which  store  it,  after  partial 
digestion,  as  honey.  While  the 
floral  glands  are  being  explored  for 
nectar,  the  visitors  become  dusted 
with  pollen  and  transfer  this  to  ripe 
stigmas  of  the  same  or  other  flowers, 
thus  insuring  pollination  in  many 
cases  where  otherwise  it  might  not 
occur  (see  Part  III  on  pollination). 
The  role  of  extrafloral  nectar  is  not 
clear.  Digestive  glands,  most  defi- 
nite in  insectivorous  plants  (p.  386),  secrete  enzymes  (p.  399)  by  which  the  soft  parts 
of  captured  insects  are  dissolved.  Essential  oils  (p.  413)  sometimes  prevent  plants 
from  being  eaten  by  animals. 


Fig.  637.  — Oil  receptacle  (a)  in  orange  (Cit- 
rus Aurantium),  formed  partly  by  splitting,  but 
chiefly  by  destruction  of  secreting  cells  and  their 
neighbors  (t) ;  0,0,  drops  of  essential  oil. — 
After  Tschirch. 


THE    MATERIAL   OUTGO   OF    PLANTS  341 

3.    THE    MOVEMENT    OF    WATER 

Transpiration  stream.  —  In  the  two  foregoing  sections  it  has  appeared 
clearly  that  the  region  where  water  enters  a  plant  and  the  region  whence 
it  leaves  are  rarely  identical,  but  that  these  parts  arc  more  or  less  widely 
separated.  There  must  be,  therefore,  movement  of  wakr  through  the 
body.  Small  quantities  of  water  are  used  in  the  body  for  saturating 
new-made  materials  and  parts,  and  for  food  making  by  green  plants. 
Somewhat  larger  quantities  arc  exuded  by  guttation,  bleeding,  or  secre- 
tion. But  the  dominant  cause  of  movement  is  to  be  found  in  evaporation, 
for  the  amount  thus  leaving  the  body  is  often  many  times  greater  than 
all  other  quantities  combined.  So  considerable  is  it  that  the  How  through 
the  body  is  figuratively  known  as  the  transpiration  stream. 

Transfer  in  small  plants.  —  In  the  smaller  land  plants,  whose  bodies 
are  composed  of  living  cells  throughout,  as  in  many  liverworts  and 
mosses,  the  water  has  to  travel  but  a  short  distance,  and  the  movement 
can  be  osmotic  only.  Evaporation  at  an  exposed  surface  concentrates 
the  solutions  in  those  cells,  thereby  reducing  the  internal  pressure  of  the 
water,  which  moves  from  an  adjacent  cell  to  reestablish  equilibrium, 
and  so  the  disturbance  soon  reaches  the  surface  cells  in  contact  with  free 
water,  which  enters  the  plant. 

Origin  of  a  conducting  system.  —  We  might  infer  that  these  osmotic 
movements  are  too  slow  to  afford  a  proper  supply  to  larger  plants,  be- 
cause, as  an  actual  fact,  they  are  in  operation  for  only  relatively  short 
distances;  the  larger  the  plant  and  the  more  necessary  a  large  supply  of 
water,  the  more  perfect  and  extensive  becomes  the  special  system  of 
tissues  for  conducting  water  by  avoiding  osmotic  transfer.  This  is 
especially  striking  when  one  follows  the  development  of  such  a  plant  as 
a  sunflower  from  the  embryo,  a  stage  when  there  is  no  water-conducting 
tissue,  to  maturity,  observing  how  the  extent  and  amount  of  this  tissue 
increases  as  the  foliage  develops  and  so  increases  the  evaporating  sur- 
face. It  may  be  assumed  that  somewhat  similar  has  been  the  history  of 
the  evolution  of  land  plants.  As  the  early  aquatics  became  more  and 
more  exposed  to  evaporation,  there  probably  came  about  the  develop- 
ment of  structures  which  limit  the  water  loss,  and  simultaneously  the 
development  of  the  water-conducting  strands,  which  greatly  facilitate 
water  movement. 

Elongation  of  cells.  —  Presumably  one  of  the  simplest  expedients  to 
accelerate  movement  is  to  reduce  the  number  of  membranes  which  the 


342 


PHYSIOLOGY 


water  must  pass  osmotically.  This  could  be  accomplished  to  a  certain 
extent  by  elongating  the  cells  in  the  main  direction  of  travel;  and  it  may 
be  that  elongation  of  the  cells  was  one  of  the  early  steps  in  the  evolution 
of  a  conducting  system.  To-day  there  are  plants  in  which  such  strands 
exist,  as  in  the  stalk  of  the  sporophyte  of  liverworts  and  mosses,  and 
these  are  often  accounted  rudimentary  conducting  tissues.1 

Lignified  traeheids.  — The  complete  elimination  of  the  cytoplasmic 
membranes  may  well  have  been  a  second  step  in  evolution.  This  would 
make  movement  more  easy  by  removing  just  so  much  resistance  from 
the  path.  If  in  addition  the  walls  were  altered  so  as  to  be  more  freely 
permeable  to  water,  movement  would  thus  be  further  facilitated.  That 
change,  known  as  lignification,  is  indeed  common.  Then  by  thickening 
the  wall  only  in  parts,  leaving  the  rest  thin,  passage  of  water  through  it  by 
way  of  the  thinner  areas  would  be  still  easier.  Strands  of  elongated  cells 
of  this  sort  constitute  the  endings  of  the  conducting  system  in  the  leaves 
of  almost  all  plants,  and  they  form  almost  the  whole  of  the  characteristic 
wood  in  gymnosperms  and  the  conducting  strands  in  pteridophytes. 

Tracheae.  —One  further  step  attains  the  condition  in  the  most  per- 
fectly developed  conducting  tissues,  namely,  the  resorption  of  the  greater 
number  of  the  transverse  partition  walls  between  the  elements,  forming 
cell  fusions  of  great  length,  known  as  ducts,  or  vessels,  or  tracheae,  the 
latter  from  their  occasional  resemblance  to  the  human  trachea  and  the 
air  tubes  of  insects.  Resorption  does  not  usually  occur  near  the  endings 
of  the  strands  in  the  leaves;  and  in  gymnosperms  it  fails  except  in  the 
primary  strands.  But  the  other  changes  do  occur,  and  the  elements 
being  cells  and  not  cell  fusions  are  distinguished  as  traeheids.  Following 
the  history  of  any  row  of  cells  which  is  to  become  a  duct,  there  is  first  the 
elongation  of  the  cells;  then  the  unequal  thickening  of  the  wall  and  its 
lignification,  together  with  the  resorption  of  most  of  the  end  walls;  and 
finally  the  disappearance  of  the  protoplasm.  Some  such  steps  as  these 
may  also  have  marked  the  evolution  of  the  conducting  system  through 
earlier  ages. 

Xylem.  — The  conducting  system  in  the  larger  plants  now  consists 
of  a  series  of  strands  known  as  xylem  strands  or  as  the  xylem  regions  of 
the  vascular  bundles  (p.  242).  Physiologically  it  is  more  satisfactory  to 
treat  the  xylem  as  independent  of  the  phloem  (p.  242),  for  although  they 
are  usually  closely  associated  in  their  course,  they  may  be  independent, 
and  the  functions  of  the  two  are  quite  unlike.    The  xylem  strands  form  a 

1  This  is  based  too  much  on  analogy  and  inference;  the  experimental  evidence  is  weak. 


TIIK   M  VI  i:KI AL   OUT(H)   OF    PLANTS 


343 


connected  >eries,  extending  from  the  tool  hair  region  t<>  the  mesophyU  of 
the  leaves,  among  which  they  branch  so  extensively  thai  there  is  s<  an  ely 
a  cell  which  is  separated  from  a  strand  by  more  than  a  half  dozen  of 
its  neighbors. 
Here  the  first 
branches  end 
blindly  (fig. 
638)  or  join 
their  fellows.  A 
section  of  the 
root  in  the  root- 
hair  region 
shows  likewise 
that  only  a  few 
cells     intervene 

between  the  free  surface  and  the  young  xylem  strands,  which,  nearer 
the  root  tip,  are  being  differentiated  from  the  plerome  (p.  239).    Like- 


Fig.  638.  —Ending  of  a  xylem  strand  among  the  cells  of  the 

mesophyU  in  a  leaf  of  lilac  (Syringa  vulgaris)  :   I,  trachcid  ;  i,  in- 
tercellular space. 


Fig.  630.  —  Skeletonized  cdpe  of  a  leaf  of  a  Finis,  shoving  the  mode  of  branching  of 
the  smaller  ribs ;  the  smallest  are  completely  gone.  —  From  a  photograph  by  Land. 

wise,  a  section  of  the  leaf  (fig.  627,  p.  319)  shows  the  relations  of  this 
water-conducting  tissue  to  the  surface,  and  an  examination  of  the  vena- 
tion of  various  leaves  (of  which  only  the  larger  veins  are  visible  to  the 


344 


PHYSIOLOGY 


unaided  eye)  shows  how  extensive  is  the  branching  (fig.  639).  Be- 
tween these  extremes  the  bundles  run,  with  lateral  connections  here 
and  there,  especially  at  the  nodes,  and  more  or  less  variation  in  size 
and  branching. 

Tracheal  markings.  — The  walls  of  the  tracheae  are  always  peculiarly 
thickened,  the  thick  regions  being  in  the  form  of  rings,  or  spirals,  or  a 
network  (figs.  640,  641).    The  thin  parts  may  be  more  extensive  than  the 

thick,  as  in  annular  and  spi- 
ral tracheae  (figs.  640,0,5; 
641,  s);  or  they  may  be 
mere  spots  in  the  midst  of 
the  thick  wall,  as  in  pitted 
tracheae  (figs.  640,  p;  641, 
p, ;-).  The  thick  and  thin 
parts  in  adjacent  tracheae  or 
tracheids  correspond;  and 
thus  the  movement  of  water 
laterally,  when  conditions 
require   it,  is  facilitated. 

In  scalariform  tracheids  the 
parts  of  the  wall  not  thickened 
are  resorbed,  and  the  neighbor- 
ing cavities  communicate  freely. 


W2     u    3      s         3  p       y       C 

Fig.  640.  —  Longitudinal  section  (diagrammatic) 
of  a  young  xylem  strand  :  c,  cambium  ;  y,  young 
trachea,  undifferentiated  except  as  to  size  ;  p,  pitted 
trachea;  5,  s,  s,  spiral  tracheae;  a,  annular  trachea; 
m,  pith.  —  After  Haberlandt. 


If  water  in  which  some 
cinnabar  has  been  rubbed 
up  be  passed  through  filter 
paper,  to  remove  all  but  the  very  finest  particles,  and  then  the  fil- 
trate is  driven  under  pressure  through  a  piece  of  fresh  pine  wood,  the 
pits  become  choked  with  cinnabar,  showing  that  water  filters  through 
them  more  easily  and  so  in  greater  quantity  than  elsewhere. 

Secondary  thickening. — The  primary  xylem,  i.e.  that  differentiated 
from  the  young  tissue  near  the  growing  points  (fig.  642),  is  adequate  to 
supply  only  the  first  leaves.  As  with  age  the  foliage  increases,  each 
primary  xylem  strand  may  undergo  secondary  thickening;  i.e.  it  has 
added  to  it  similar  tissues,  originating  from  a  layer  of  dividing  cells 
which  adjoins  its  outer  face  (fig.  643).  In  addition,  this  meristem  (cam- 
bium), arising  between  the  primary  strands,  may  originate  new  strands 
of  xylem  tissue  between  the  primary  ones.  These  secondary  strands 
may  then  increase  in  thickness  in   the  same  manner  as  the  primary 


THE    MATERIAL    ()UT(!()    OF    PLANTS  345 


f  —      s     •"       -  ■     r 

Fig.  641.  —  Enlarged  details  of  spiral  (s),  pitted  (/>),  and  reticulate  (r)  tracheae;  at 
d,  traces  of  original  partition  walls.  — Adapted  from  IIabeklanut  and  TSCHERCH. 

ones.     When  numerous  primary  and  secondary 
strands  are  produced,  they  may  form  a  column 

of  xylem,  with  pith  in 

the  center,  interrupted 

by  thin  radiating  plates 

of     parenchyma,     the 

pith  rays.     Such  is  the 

condition    in   the    sun- 
flower, castor  bean  (fig. 

644),  and    many   other 

di(  otyledons. 

In    case    the    xylem 

strands  do  not  undergo 

individual  secondary 
thickening  (as  is  the  case  in  most  monocoty- 
ledons), there  may  be  a  cylinder  of  meristem 
which  repeatedly  produces  new  bundles,  as  in 
asparagus.  But  in  all  plants  which  produce 
numerous  leaves  the  increasing  evaporation  is 


Fig.  642.  —  Young  vas- 
cular bundle:  />,  primary 
phloem  ;  .v,  primary  xylem  ; 
r,  first  divisions  of  cambium 
cells. — After  Bonnikk. 
Diagrammatic. 


Fig.  643.— Older  vas- 
cular bundle,  with  second- 
ary thickening  in  , 
p,  phloem  ;  e,  cambium, 
forming  by  division  both 
secondary  phloem  and 
xylem;  .v,  xylem,  com- 
post '1  of  .Vj  and  .v.,  the 
primary  and  secondary 
xybm.  —  Ai't.-r   Bonnier. 


346 


PHYSIOLOGY 


accompanied  by  an  increase  of  the  conducting  tissues  (see  Part  I,  p.  243). 
Annual  thickening.  —  In  trees  and  shrubs  the  xylem  undergoes  sec- 
ondary thickening  in  the  first  season  of  growth,  and  this  is  resumed  in 
the  second  season,  and  so  on,  from  the  persistent  cambium.  Thus  arises 
a  great  cylinder  of  xylem,  which  constitutes  the  wood  of  the  trunk  and 


Fig.  644. —  Cross  section  of  stem  of  Ricinus  communis,  showing  ring  of  secondary 
xylem;  for  description,  see  fig.  541.  —  From  Part  I. 

branches.  In  many  trees  the  xylem  formed  in  the  course  of  the  growing 
season  gradually  changes  its  character.  The  first  formed  tissues  con- 
tain many  large  ducts  and  less  mechanical  tissue,  while  the  later  formed 
xylem  has  small  ducts  and  much  mechanical  tissue.  In  these  cases  the 
open  tissues  produced  in  the  spring  abut  on  the  denser  ones  last  pro- 
duced in  the  summer  or  autumn,  and  the  sharp  contrast  marks  visibly 
the  periodicity  in  growth.  As  these  differences  in  the  tissue  depend  upon 
growth,  and  as  this  is  most  affected  by  the  annual  seasonal  changes,  the 
growth  rings  are  usually  annual  rings,  and  make  possible  an  estimate  of 


THE    MATERIAL   OUTGO   I  >l     PLANTS 


347 


the  age  of  the  tree.  But  annual  rings  may  show  subordinate  rings,  due 
to  some  pronounced  climatic  change  whi<  h  affe<  ted  the  rale  of  growth 
more  than  once  in  the  year.  These  rings  may  be  so  pronounced  as  to 
make  the  age  estimate  uncertain,  but  in  temperate  regions  the  annual 
rings  are  usually  well  defined.  In  some  trees  the  differences  between 
spring  and  autumn  wood  are  slight,  and  the  annual  rings  are  discerned 
with  more  difficulty.  The  definite  annual  rings  are  responsible  in  large 
part  for  the  "  grain  "  of  wood.     (See  also  Pari  III  on  annual  rings.) 

Heart  wood  and  sap  wood.  —  With  age  the  xylem  loses  its  capacity 
to  conduct  water,  and  sooner  or  later  may  so  change  in  color  and  com- 
position as  to  distinguish  the  older  heart  wood  from  the  newer  sap  wood. 
These  changes,  however,  do  not  coincide  with  the  annual  rings,  nor  do 
they  exactly  correspond  with  the  differences  in  conductivity,  since  in 
some  plants  the  whole  of  the  sap  wood,  hut  in  others  only  the  youngest 
portion  of  it,  is  traversed  by  the  transpiration  stream. 

Xylem  is  water  path.  — The  evidence  that  the  xylem  is  the  path  of  the 
transpiration  stream  rests  in  part  upon  direct  observation,  hut  mainly 
upon  inference  from  the  effects  of  cutting  the  xylem  strands  or  hloi  king 
the  tracheae. 

Relative  development.  —  In  the  first  place,  one  finds  a  general  relation 
between  the  amount  of  transpiration  and  the  development  of  the  xylem. 
In  most  submersed  water  plants  the  xylem  is  very  poorly  differentiated, 
its  place  being  occupied  by  some  elongated  cells,  slightly  different  from 
their  neighbors,  which  are  morphologically  equivalent  to  xylem,  but 
physiologically  they  are  negligible.  On  the  other  hand,  in  climbing 
plants,  whose  spread  of  foliage  is  large  and  their  stems  slender,  the  xylem 
reaches  its  best  development,  occupying  a  large  proportion  of  the  cross 
section  of  the  stem,  and  having  ducts  of  relatively  large  diameter.  Not 
much  reliance  could  he  placed  upon  such  a  loose  and  general  relation, 
were  it  not  for  more  direct  evidence. 

Girdling. — Girdling  experiments  show  more  clearly  the  path  of  the 
water.  It  is  a  matter  of  common  knowledge  that  by  tutting  through 
the  sap  wood  of  a  tree  the  foliage  promptly  wilts  and  dies;  and  in 
earlier  days  it  was  commoner  than  now  to  see  the  trees  in  some  piece  of 
woodland  "girdled,"  preparatory  to  clearing  the  ground  for  cultivation. 
But  removing  only  the  hark  does  not  produ<  e  wilting,  except  after  weeks 
or  months,  for  thus  only  the  phloem  strands  an-  interrupted.  More 
exact  experiments  may  he  performed.  By  selecting  a  herbaceous  plant 
whose  vascular  bundles  are  distinct,  one  may  cut  the  pith,  the  vascular 


348  PHYSIOLOGY 

bundles,  and  the  cortex  in  different  specimens  and  compare  the  effect. 
It  will  be  found  that  only  in  the  specimens  whose  bundles  have  been 
cut  do  the  leaves  wilt,  and  the  fact  that  in  woody  plants  the  bark  may  be 
removed  without  causing  wilting  eliminates  the  phloem  strands.  Such 
experiments  permit  the  inference  only  that  the  xylem  strands  are  the 
chief  paths  of  the  transpiration  stream,  not  that  they  are  the  sole  path ; 
for  wilting  implies  merely  an  inadequate  water  supply. 

Water  moves  in  the  lumina.  —  But  the  path  can  be  localized  more 
exactly.  A  shoot  of  a  climber,  such  as  Clematis,  may  be  cut  off  under 
water,  and  the  end  sliced  very  obliquely,  so  as  to  open  wide  the  ends 
of  the  ducts.  If  this  shoot  be  fastened  to  a  microscope  slide,  and  the  end 
covered  with  water,  into  which  has  been  introduced  some  finely  divided 
carbon,  as  from  Chinese  ink,  one  may  watch  the  water  swirling  into  the 
open  ends  of  the  ducts,  its  course  being  made  evident  by  the  opaque 
particles  it  carries.  Under  such  circumstances  it  is  evident  that  the  water 
enters  and  probably  traverses  the  lumen  of  the  trachea.  But  this  was 
for  a  long  time  a  disputed  point.  When  the  extraordinary  freedom  of 
movement  of  water  in  lignified  tissues  was  discovered,  it  was  held  that 
the  water  traveled  in  the  substance  of  the  walls  and  not  in  the  lumina 
(the  chambers  they  enclose).  This  opinion,  however,  rested  upon  inac- 
curate experimentation. 

Closing  the  lumina.  —  Attempts  were  made  by  compressing  the  stem  in  a  vise  to 
collapse  the  tracheae,  and  so  to  close  their  lumina.  In  the  earlier  experiments  of 
this  sort,  wilting  did  not  occur,  and  the  inference  was  plain,  therefore,  that  the  water 
traveled  in  the  wall  itself.  Repeated  studies  showed  that  the  difficulty  of  compress- 
ing the  tracheae  had  been  underestimated,  and  that  when  they  were  actually 
closed  mechanically,  the  leaves  did  wilt.  A  better  method  of  closing  them  is  by 
plugging  them  with  paraffin  or  gelatin  which  melts  at  a  low  temperature.  By 
cutting  a  shoot  under  the  melted  material,  it  is  carried  up  instantly  to  some  distance 
in  the  tracheae.  When  cooled,  it  solidifies  and  a  fresh  surface  of  wall  can  be  exposed 
by  removing  a  thin  slice,  while  the  lumina  remain  plugged.  The  leaves  of  such  a 
shoot  promptly  wilt  when  exposed  to  dry  air. 

Path  of  least  resistance.  —  On  the  whole,  therefore,  it  is  fairly  certain 
that  the  transpiration  stream  traverses  the  xylem  strands,  and  that  it  is 
the  lumina  of  the  tracheae  that  form  the  chief  conduits  for  the  water. 
That  some  travels  in  the  walls  is  quite  probable,  especially  when  the 
tracheae  are  partly  blocked,  as  they  often  are,  by  gas,  the  path  of  least 
resistance  being  followed  here  as  always.  Nor  is  it  impossible  that 
some  water  moves  in  the  cortex;  but  this  is  never  enough  to  cover  any 
considerable  loss  by  evaporation. 


THE   MATERIAL   OUTGO   OF    PLANTS 


349 


Ascent  of  water.  —  As  to  the  forces  concerned  in  the  ascent  of  water, 
little  that  is  definite  can  be  said,  for  the  problem  is  one  of  extraordinary 
complexity,  and  knowledge  of  the  exact  physical  conditions  is  very 
difficult  to  attain.  Nor  is  it  likely  that  the  problem  i  ould  be  solved  wen- 
all  the  factors  in  the  plant  body  known,  simply  for  lack  of  knowledge 
of  the  physical  principles  involved. 

Capillarity.  —  Some  "  causes  "  frequently  assigned  and  popularly 
current  may  be  definitely  discarded.  The  first  of  these  is  capillarity, 
as  commonly  understood.  The  xylem  ducts  are  narrow  tubes.  Water 
rises  in  capillary  glass  tubes  above  the  level  outside,  and  the  smaller  the 
bore  the  higher  it  rises.  Oil  rises  in  a  twisted  lamp  wick  by  capillarity. 
What  more  simple  than  to  "  explain  "  the  rise  of  water  in  the  ducts  of 
the  xylem  strands  by  ascribing  it  to  capillarity,  since  here  are  "  strands  " 
and  "  tubes"?  But  surface  tension  (which  is  a  better  name  for  capil- 
larity) implies  a  free  surface,  and  within  the  duct  there  can  be  no  free 
surface  which  is  lifting,  as  in  an  open  glass  tube.  If  one  appeals  to  the 
surfaces  bounding  the  bubbles  of  gas  so  common  in  tracheae  (see  p.  350), 
it  must  be  remembered  that  for  every  meniscus  concave  upwards  there  is 
one  concave  downwards  to  balance  it.  Nor  can  one  neglect  the  numer- 
ous transverse  walls  in  the  xylem  of  angiosperms,1  and  the  fact  that  all 
the  effective  xylem  of  gymnosperms  is  composed  of  tracheids.  How- 
surface  tension  forces  may  operate  at  the  evaporating  surfaces  in  the 
leaves  is  not  known;  but  these  are  not  the  ones  referred  to  when  capil- 
larity is  invoked  as  the  cause  of  the  ascent  of  water,  or  at  least  an  aid  to 
it. 

Root  pressure.  —  Root  pressure  (see  p.  336)  is  frequently  alleged  to  be 
active  in  forcing  water  up;  and  it  is  even  held  to  be  adequate  in  the  case 
of  the  herbaceous  plants  and  low  shrubs,  though  confessed  to  be  insuf- 
ficient in  the  taller  trees.  The  radical  difficulty  with  turgor  in  the  root 
cortex  as  a  cause  of  the  ascent  of  water,  or  at  least  an  aid  to  it,  is  that  it 
does  not  exist  when  it  is  most  needed.  In  the  very  nature  of  the  case  the 
root  cortex  can  be  fully  turgid  only  when  it  has  an  abundance  of  water; 
and  it  is  not  likely  to  have  that  when  evaporation  is  active.  To  develop 
root  pressure  it  is  necessary  to  check  evaporation,  as  by  decapitation, 
and  only  after  a  time  does  water  begin  to  ooze  from  the  xylem  in  conse- 
quence of  turgor.  ( >ften  water  at  first  enter-  the  stump  of  a  da  apitated 
plant,  showing  clearly  that  there  was  no  surplus  of  water  under  previous 

1  The  longest  continuous  ducts  found  exceed  5  m.,  but  those  1  m.  long  are  ran-,  and 
the  average  is  probably  less  than  10  cm. 


3  so  PHYSIOLOGY 

conditions.  Nor  can  root  pressure  be  invoked  even  as  an  aid.  For 
unless  maximum  turgor  can  be  attained  no  extrusion  of  water  from 
cortical  cells  is  possible. 

If  a  boy  could  push  a  wagon  while  the  horse  walked,  he  would  be  unable  to  push 
as  soon  as  the  horse's  speed  exceeded  his  own.  If  he  clung  to  the  wagon,  he  would 
be  merely  a  drag,  though  if  he  ran  he  would  be  less  of  a  drag  than  if  he  made  no 
exertion.  The  transpiration  horse  often  goes  too  fast  with  the  water  wagon  for  the 
root  pressure  boy  to  push.  Then  his  grip  is  broken  at  once  and  he  is  no  drag  on 
the  load,  for  root  pressure  cannot  even  hold  on  like  the  boy  and  "  help  "  by  not 
being  wholly  a  drag. 

Atmospheric  pressure. — Atmospheric  pressure  has  been  invoked  as 
an  explanation.  It  is  found  that  the  gases  which  develop  in  the  tracheae 
are  often  under  a  pressure  less  than  one  atmosphere.  Indeed  they 
develop  there  readily  because  this  is  the  case.  The  tracheae,  it  must 
be  remembered,  are  dead  cells;  their  lumina  therefore  are  as  free  to  be 
occupied  by  gases  as  are  intercellular  spaces.  Whenever  the  concen- 
tration of  gases  dissolved  in  a  free  liquid  exceeds  the  amount  normal 
at  one  atmosphere  pressure,  the  gas  particles  escape  from  solution  and 
form  bubbles. 

This  happens  when  any  bottle  of  liquid  "  charged  "  with  CO2  is  opened.  The 
gas  is  dissolved  in  the  liquid  under  a  pressure  greater  than  one  atmosphere;  on  un- 
corking it  the  pressure  is  reduced  immediately  to  that  of  the  outer  air,  the  gas  flashes 
at  once  into  bubbles,  and  portions  of  the  liquid  are  often  forced  out  of  the  bottle 
by  the  violence  of  effervescence. 

Bubbles  would  inevitably  form  in  the  water  of  the  tracheae,  whenever 
that  water  has  the  pressure  on  it  reduced  below  one  atmosphere.  If  this 
pressure  were  equal  to  half  an  atmosphere,  it  is  argued  that  such  tension 
could  "  lift  "  water  about  5  m.  So  it  could,  if  the  lower  end  of  the  water 
columns  were  open  to  the  pressure  of  the  atmosphere  and  there  were  no 
resistance.  If  one  took  away  half  an  atmosphere  of  pressure  from  the 
upper  end  of  a  water  column  and  left  a  whole  atmosphere  of  pressure  to 
act  on  the  lower  end,  of  course  the  water  would  rise  to  the  point  of  equi- 
librium. But  these  conditions  do  not  exist  in  the  plant.  Evaporation 
may  reduce  the  pressure  on  the  water  in  the  tracheae,  but  the  lower  end 
of  the  water  column  is  not  open.  The  living  cells  of  the  root  cortex  are 
interposed,  and  water  cannot  be  driven  through  them  by  a  difference  of 
half  an  atmosphere  or  even  a  whole  atmosphere  of  pressure;  nor  has 
the  pressure  in  the  tracheae  ever  been  found  to  fall  to  zero.  If  it 
were  zero,  and  there  were  no  resistance  to  the  movement,  water  could 
be  pressed  up  to  a  height  of  only  10  m.,  a  small  fraction  of  the  100  m. 


I  ill.    MATERIAL   *  HJTGO   OF    PLANTS  351 

which  the  tallest  trees  attain.  Atmospheric  pressure  therefore  is 
utterly  inadequate  at  best.  The  most  that  can  be  allowed  is>  this: 
by  how  much  the  difference  in  atmospheri*  pressure  in  the  tracheae 
and  in  the  air  tends  to  make  it  easier  for  water  to  pas>  through  the  root 
hair  and  the  root  cortex,  by  so  much  atmospheri<  pressure  may  be  said 
to  help  in  the  entry  of  water.  But  the  very  fact  thai  these  differences 
exist  shows  that  they  are  not  compensated  by  the  movement  of  the 
water.  In  fad  the  difference  between  inner  and  outer  pressure  seems 
to  be  rather  a  result  than  a  cause  of  water  movement. 

Role  of  living  cells.  — The  ultimate  cause  of  the  ascent  of  sap  is  tran- 
spiration; but  how  it  acts  is  entirely  unknown.  The  energy  employed  in 
vaporizing  the  water  is  adequate  to  lift  it  miles  high;  but  how  i>  it  ap- 
plied so  as  to  keep  a  continuous  stream  rising? 

One  link  in  the  chain  is  the  osmotic  relations  of  the  living  cells  of  the 
leaf;  for  if  the  leaves  be  killed,  evaporation  continues  from  their  cells, 
but  the  supply  from  the  xylem  strands  is  interrupted  and  the  leaf  dries 
up  promptly. 

It  was  also  proposed,  first  many  years  ago,  to  ascribe  the  ascent  of 
water  to  the  action  of  living  cells  along  the  course  of  the  xylem  strands, 
and  this  theory  is  being  advocated  again  to  day.  One  notion  of  their 
action  was  that  it  is  like  that  of  relay  pumps,  which  take  water  in  at  one 
level  and  force  it  up  to  a  higher  level.  It  is  difficult  to  conceive  the 
physics  of  such  an  operation,  and  there  is  no  anatomical  evidence  of  such 
a  mechanism,  unless  the  cells  of  the  pith  rays  are  the  active  cells.  The 
experimental  evidence  as  to  the  cooperation  of  live  cells  in  the  process 
is  contradictory,  to  say  the  least,  and  by  its  very  nature  the  theory  must 
be  rather  vague.  That  the  living  cortex  and  wood  parenchyma  are  neces- 
saryto  keep  the  xylem  in  proper  condition  for  conduction  is  assumed. 

Cohesion  theory.  —  A  current  theory,  which  also  is  confronted  by 
many  difficulties  and  leaves  much  to  be  explained,  is  based  upon  the 
fact  of  the  cohesion  of  water.  That  serins,  at  first  blush,  like  talking  of 
the  strength  of  a  rope  of  sand;  but  it  is  actually  very  difficult  to  break 
a  small  column  of  water,  if  sidewise  or  shearing  strains  are  eliminated. 
I'hc  cohesive  strength  of  water  is  variously  estimated  by  physicists  at 
10-150  atmospheres. 

The  rupture  of  sporangia  <>f  ferns  and  the  anthers  of  flowering  plants,  and  the 
collapse  of  cells  on  drying,  have  now  been  shown  to  depend  upon  the  cohesion  of 
water.  The  mechanism  for  spore  scattering  in  the  sporangium  >>f  a  fern,  for  ex- 
ample, is  illuminating.     It  consists  of  thi<  k  walled  1  ells  around  the  edge,  the  annul  us 


352 


PHYSIOLOGY 


(a,  figs.  645-647),  which  contain  water.  As  the  water  evaporates  it  pulls  the  cell 
walls  together,  and  in  doing  so  straightens  the  ring  and  tears  open  the  weak  side. 
The  thick  elastic  C~shaped  walls  of  the  cells  resist  this  compression,  until  finally 
the  cohesion  of  water  in  the  wall  with  the  free  water  in  the  lumen  is  overcome,  and 
the  sudden  elastic  recoil  of  the  annulus  hurls  the  spores  as  from  a  sling. 


Figs.  645-647.  —  Rupture  of  sporangium  of  a  fern  (Polystichum  acrostichoides):  645, 
the  sporangium  cracked;  a,  the  annulus;  646,  position  of  complete  reversion,  many  of 
the  spores  adherent  to  the  upper  part  of  the  sporangium;  647,  position  after  recoil,  the 
sporangium  emptied;  dotted  lines  in  this  figure  show  the  position  as  in  646.  —  After 
Atkinson. 

This  cohesion  is  predicated  of  the  columns  of  water  which  occupy 
the  tracheids  and  tracheae  of  the  xylem,  and  it  is  coherent  even  through 
the  end  and  side  partitions  (see  theory  of  relation  of  water  and  cell  wall, 
p.  301).  If  now  any  adequate  lifting  force  could  be  applied  at  the  upper 
end,  the  cohesion  of  the  water  is  sufficient  to  enable  it  to  hold  together 
even  to  the  roots  of  the  tallest  trees.  That  lifting  force  is  evaporation, 
and  the  osmotic  relations  of  water  in  the  live  cells  of  the  leaf  furnish  the 
connection.  Why  the  water  columns  do  not  break  wherever  bubbles 
of  gas  appear  (and  they  must  appear  whenever  the  column  is  under  any 
considerable  strain),  is  not  satisfactorily  explained;  and  other  like 
difficulties  appear.  Yet  this  theory  at  least  faces  in  the  right  direction, 
seeking  to  give  an  account  of  the  rise  of  water  in  purely  physical  terms. 
However,  as  this  phenomenon  has  baffled  investigators  for  more  than  a 
century,  it  may  be  a  long  while  before  it  can  be  satisfactorily  described. 


4.    OTHER    LOSSES 

Gases  from  the  shoot.  — Quite  apart  from  the  liquids  and  water  vapor 
which  escape  from  the  aerial  parts,  there  are  gases  which  are  constantly 
set  free  and  leave  the  plant  as  such.  These  are  carbon  dioxid  and 
oxygen;   the  former  is  one  of  the  usual  end  products  of  respiration,  and 


THE    MATERIAL   OUTGO   OF    PLANTS  353 

the  latter  is  a  by-product  of  food  making,  bul  is  used  by  all  live  parts  in 
respiration.  Carbon  dioxid  i>  continually  produced  in  all  live  part-; 
but  in  green  parts,  when  adequately  lighted, it  <  an  be  used  for  making 
food,  and  therefore  in  these  parts  under  such  conditions  it  never  accu- 
mulates to  an  amount  which  permits  it  to  diffuse  out.  Oxygen  is  only 
intermittently  produced.  When  the  green  parts  are  making  certain 
foods,  its  production  is  a  measure  of  their  activity;  l>ut  that  takes  plat  e 
only  in  the  light.  Since,  therefore,  the  leaves  are  the  green  parts  par 
excellence,  oxygen  escapes  chiefly  from  them,  because  the  amount  pro- 
duced is  in  excess  of  that  used  in  their  respiration.  When  it  has  accu- 
mulated in  the  cell  sap  to  a  concentration  whose  osmotic  pressure  is 
greater  than  its  pressure  in  the  air  (i.e.  about  0.2  of  an  atmosphere,  or 
152  mm.  of  mercury),  it  will  fly  off  as  a  gas  from  the  surface  of  the  cell  into 
the  internal  atmosphere  of  the  aerating  system.  Likewise  when  carbon 
dioxid  has  accumulated  to  a  suitable  pressure  (less  than  0.0003  A.,  or 
about  0.22  mm.  Hg.),  it  begins  to  diffuse  into  the  air. 

Diffusion  from  the  root.  — Oxygen  can  be  formed  only  in  green  parts 
and  hence  escapes  only  from  aerial  parts;  carbon  dioxid,  being  formed 
in  all  live  cells,  can  also  escape  through  the  other  permeable  region,  the 
root.  Its  escape  there  may  be  directly  into  the  soil  water,  whenever  it 
has  accumulated  to  a  greater  pressure  in  the  cell  sap.  To  demonstrate  dif- 
fusion it  is  only  necessary  to  grow  the  roots  in  contact  with  a  polished  mar- 
ble plate  (calcium  carbonate),  whose  surface  will  be  etched  along  the  lines 
of  contact  because  water,  "  carbonated  "  by  the  C02  escaping  from  the 
roots,  converts  the  calcium  carbonate  (CaC03)  into  calcium  bicarbonate 
[Ca(HC03)2],  which  is  readily  soluble.  Or  by  growing  seedling  in 
water  with  phenolphthalein  (an  indicator  which  is  rose  red  in  weak 
alkaline  and  colorless  in  acid  solution),  the  water  will  be  decolorized  by 
the  roots;  but  the  color  will  return  upon  boiling,  thus  driving  off  the 
COj  which  had  united  with  the  indicator.  Were  any  mineral  or  organic- 
acids  the  cause  of  the  decoloration,  the  color  would  not  return. 

I'.ui  l>esidesC02  other  substances  may  l.avc  the  planl  by  wayof  the  roots.  At 
present  these  are  not  accurately  known.  Water  i  ultures  made  with  soil  extra*  ts 
indicate  that  organic  compounds,  often  very  deleterious  to  the  culture  plants,  are 
frequently  present.  These  may  have  come  into  the  soil  by  diffusion  from  roots 
(see  p.  315).  Acid  salts,  such  as  hydrogen  potassium  phosphate,  are  probably 
not  among  the  exudates,  as  once  they  were  believed  to  be.  Yel  any  substani  e  in 
the  root  cortex,  to  which  the  cells  are  permeable,  may  ea  ape  ;  and  when  the  matter 
is  studied  further,  many  compounds,  now  unsuspected,  may  be  found  diffusing 
into  the  soil  water. 

C.  B.  &  C.  BOTANY  —  23 


354 


PHYSIOLOGY 


Mechanical  losses.  — Mechanical  losses  must  also  be  taken  into  ac- 
count. In  all  plants  the  drying  of  leaves,  flower  parts,  rootlets,  and 
even  larger  parts  of  the  body,  is  followed  sooner  or  later  by  their  falling 
off.  In  annuals,  the  whole  body  perishes  at  the  end  of  the  growing 
season;  hence  the  perennials  offer  the  best  examples.  In  woody  peren- 
nials, particularly,  the  partial  fall  of  the  leaves  in  summer,  due  to  heat, 
drought,  or  other  causes,  and  the  complete  autumnal  fall,  are  striking 
losses  of  material.  Yet  this  is  not  so  expensive  to  the  plant  as  it  might 
seem  at  first  sight,  for  a  large  part  of  the  available  foods  have  been  trans- 
ferred from  the  leaves  before  their  fall,  and  what  is  left  is  chiefly  cell-wall 
stuff,  unavailable  organic  matter,  and  ash.  Nevertheless,  much  of  that 
represents  past  expenditure  of  energy  and  is  a  dead  loss;  though  by 
decay  some  of  the  materials  again  become  available  for  rebuilding. 

Fall  of  leaves.  — The  once  active  food-making  machines  go  to  the 
scrap  heap  in  autumn  and  have  no  value  except  as  junk.  Their  deterio- 
ration is  progressive.  In  the  leaves  of  woody  plants  as  compared  with 
other  parts,  there  is  with  age,  as  a  rule,  a  steady  accumulation  of  dry 
matter  and  a  rising  proportion  of  ash. 

Thus  in  the  leaves  of  the  European  beech  (Fagus  sylvatica) : 
May         June         July 
Per  cent  dry  matter  .     .     23.35       40.21      43-04 
Per  cent  of  ash      .     .     .       4.67         5.20        7.45 

In  black  locust  (Robinia  Pseudacacia): 

May 

Per  cent  dry  matter 26.50 

Per  cent  of  ash 6.25 

In  500  leaves  of  the  plane  tree  (Platanus  orirntalis): 
June  July 

Grams  dry  matter    .     .      .     142.53  184.70 

Grams  of  ash        ....         8.70  14.62 

Contrast  with  these  figures  the  average  ash  content  of  the  wood  of  such  trees, 
which  is  about  0.7  per  cent,  with  a  minimum  of  0.2  per  cent  and  an  occasional 
maximum  of  about  3  per  cent. 

This  high  ash  content  of  leaves  is  not  due  merely  to  the  retention  of 
mineral  matters  when  the  water  evaporated,  as  lime  scale  accumulates 
in  a  tea  kettle.1  Rather  the  using  of  certain  constituents  of  the  salts, 
particularly  the  nitrogen,  sulfur,  and  phosphorus,  left  behind  the  bases, 
calcium,  magnesium,  etc.,  ready  to  enter  into  new  combinations  and  to 

1  This  is  further  evidenced  by  the  fact  that  the  ribs  of  leaves  are  usually  richer  in  ash 
than  the  mesophyll. 


Aug. 

Sept.        Oct. 

Nov. 

5°-74 

47.42       40.37 

45-55 

9-03 

8.90       10.80 

11.42 

July 

Sept. 

Oct. 

35-90 

44-30 

44.60 

7-75 

8.22 

11.74 

is): 
Aug. 

Sept. 

Oct. 

182.80 

I93-85 

196.24 

17.81 

20.12 

2i-33 

THE    MATERIAL   OUTGO   OF    PLANTS  355 

reappear  in  the  ash,  when  the  organic  matter  is  burned  away,  as  Ca< », 
MgO,  etc.  Moreover, certain  mineral  salts  maybe  stored  in  the  walls, 
as  silica  often  is;   and  these  reappear  as  oxids  in  the  ash. 

Fall  of  branches.  —  In  woody  perennials  the  competition  between 
brain  hes  is  so  severe  that  many  more  die  than  survive.  Thousands  of 
rudimentary  branches  (as  buds)  never  develop  at  all,  and  other  thousands, 
after  growing  for  a  year  or  two,  are  outstripped  by  their  more  fortunately 
situated  fellows,  die,  and  drop  off.  The  mortality  is  vastly  greater  than 
is  realized  without  close  observation,  such  as  was  made  on  a  volunteer 
black  cherry,  and  described  in  figurative  language  thus: 

Tin'  first  year  it  made  a  straight  shoot  nineteen  inches  high,  which  producec 
twenty-seven  buds.  It  also  sent  out  a  branch  eight  in<  lies  long  w  hi<  li  Wore  twelve 
buds.  The  little  tree  had,  therefore,  enlisted  thirty-nine  soldiers  for  the  coming 
conflict.  The  second  year  twenty  of  these  buds  did  not  grow.  Nineteen  of  them 
made  an  effort,  and  these  produced  three  hundred  and  seventy  buds.  In  two  years 
it  made  an  effort,  therefore,  at  four  hundred  and  nine  branches,  bul  at  the  close  of 
the  sec  ond  year  there  were  only  twenty-seven  branches  upon  the  tree.  At  the  close 
of  the  third  year  the  little  tree  should  have  produced  about  thirty-live  hundred  buds 
or  branch  germs.  It  was  next  observed  in  July  of  its  fourth  year,  when  it  stood 
just  eight  feet  high  ;  instead  of  having  between  three  and  four  thousand  branches,  it 
bore  a  total  of  two  hundred  and  ninety-seven,  and  most  of  them  were  only  weak 
spurs  from  one  to  three  inches  long.  It  was  plain  that  not  more  than  twenty,  at 
the  outside,  of  even  this  small  number  could  long  persist.  The  main  stem  or  trunk 
bore  forty-three  branches,  of  which  only  eleven  had  much  life  in  them,  and  even 
some  of  this  number  showed  signs  of  weakness.  In  other  words,  in  my  little  cherry 
tree,  standing  alone  and  having  things  all  its  own  way,  only  one  bud  out  of  every 
hundred  and  seventy-live  succeeded  in  making  even  a  fair  start  towards  a  perma- 
nent branch.  And  this  struggle  must  have  proceeded  with  greater  severity  as  the 
top  became  more  complex,  had  I  not  put  an  end  to  its  travail  with  the  axe  !  — 
Bailey:   Survival  of  the  unlike,  p.  88. 

Loss  of  bark. — The  constant  flaking-off  of  bark,  when  the  warping 
due  to  wetting  and  drying  loosens  the  outer  portions,  or  the  steady 
weathering  of  the  solid  bark,  occasions  further  losses  of  a  relatively 
inexpensive  kind.  As  in  some  cases  waste  products  accumulate  in  the 
bark,  this  may  be  accounted  <>ne  way  by  which  the  plant  gets  rid  of 
wastes.     Hark  also  contains  a  very  large  percentage  of  ash. 

Fruits  and  seeds.  —  Fruits  and  seeds  are  separated  annually  from  the 
body.  These  are  loaded  with  surplus  food  for  the  embryo,  and  50  consti 
tute  a  most  expensive  loss  -  one  that  not  infrequently  distinctly  impairs 

the  vitality  of  the  plant.  The  intermittent  bearing  of  orchard  tree-. 
vines,  etc.,  may  herein  find  a  partial  explanation. 


CHAPTER   III  —  NUTRITION 
I.   THE    NATURE    OF    PLANT   FOOD 

Fool  in  general  is  organic.  — The  question,  what  is  food  for  plants, 
elicits  very  different  answers  according  to  the  point  of  view.  The  term 
food  is  not  one  which  admits  of  accurate  definition,  and  the  difficulty 
increases  the  wider  the  range  of  organisms  to  which  it  refers.  A  lion 
obviously  lives  upon  flesh,  and  the  general  constituents  of  his  food  can  be 
determined.  A  sheep  feeds  on  herbage,  and  that  can  be  analyzed.  A 
man  consumes  meat  and  vegetables  of  the  most  varied  sorts.  A  fungus 
like  PenicUlium,  which  will  grow  on  a  glass  of  jelly  or  an  orange  or  a  piece 
of  cheese  or  a  plate  of  gelatin,  obviously  feeds  upon  vegetable  or  animal 
substances  indifferently.  The  nutritive  constituents  of  flesh  and  vege- 
tables are  many  and  diverse;  plainly  the  term  which  is  to  include  them 
must  be  most  general.  That  term  by  common  consent  is  food.  It 
represents  the  totality  of  substances  which  nourish  an  organism  and 
enable  it  to  pass  successively  through  the  phases  of  its  normal  develop- 
ment. Now  all  the  substances  referred  to  belong  to  a  category  known 
as  organic,  because  they  are  all  produced  by  the  chemical  processes  in  a 
living  organism.  Food,  therefore,  for  the  lion,  the  sheep,  the  man,  the 
mold,  is  composed  of  organic  substances.  It  is  true  that  there  are  also, 
in  the  very  organic  substances  themselves  and  dissolved  in  the  juices 
which  make  part  of  them,  mineral  salts  of  various  kinds,  and  that  these 
are  indispensable  to  living  beings;  but  their  amount  is  very  small  in- 
deed, and  alone  they  are  quite  incapable  of  sustaining  life.  For  the 
present,  therefore,  they  may  be  left  out  of  account. 

Is  the  food  of  green  plants  inorganic  ?  — The  beings  enumerated 
represent  all  sorts  of  organisms  except  the  green  plant.  When  we  ask, 
"On  what  does  the  green  plant  feed?"  the  answer,  based  on  analogy, 
has  been,  "On  the  substances  that  enter  it  —  water,  mineral  salts,  and 
carbon  dioxid;  for  with  these  alone  it  can  develop  from  embryo  to 
maturity."  These  are  inorganic  substances;  and  if  the  answer  be  true, 
the  food  of  green  plants  is  inorganic  and  that  of  all  other  beings  organic. 

356 


NUTRITION  357 

Is  "  food  "  food  only  for  certain  cells?  —  The  first  thing  that  awakens 
suspi<  ion  as  ta  the  wisdom  of  this  answer  is  that  the  living  matter  of  green 
plains  is  like  that  of  all  other  living  things,  and  it  would  be  very  strange 
if  in  them  protoplasm  could  be  nourished  with  inorganic  substances, 
when  in  all  others  it  requires  organic  material.  Yet  the  green  plant 
might  be  differently  constituted;  and  it  is  said  by  way  of  explanation  thai 
this  peculiarity  is  due  to  the  presence  of  the  green  pigment,  <  hlorophyll. 
On  examining  this  point,  it  is  found  that  only  a  part  of  the  plant  has 
chlorophyll.  .Most  roots  entirely  lack  it;  only  the  outer  cells  of  the  stem 
ever  contain  it;  and  there  are  many  cells,  even  in  a  thin  leaf,  and  a  great 
mass  of  them  in  a  fleshy  leaf,  which  are  not  green.  Then  we  are  forced 
to  state  the  matter  thus:  the  green  parts  of  green  plants  use  inorganic 
"  food  ";  the  colorless  parts  require  organic  food,  for  it  is  conceded  on 
all  hands  that  the  colorless  cells  are  unable  to  utilize  any  carbon  dioxid 
and  water.  Whence  it  would  seem  that  one  cell  might  nourish  itself  with 
inorganic  "  food  "  and  its  next  neighbor  he  unable  to  do  so.  That  would 
certainly  be  a  confusing  situation  if  it  could  not  be  better  described. 

Is  "food"  food  only  at  certain  times? — It  appears,  further,  that 
carbon  dioxid  and  water  can  be  "  foods  "  only  part  of  the  time;  namely, 
when  the  green  cells  are  adequately  lighted;  So  except  in  the  day,  even 
the  green  cell  would  require  organic  food!  The  situation  would  have 
to  be  stated  thus:  The  "  food  "  of  the  green  cells  only,  and  only  by  day, 
consists  of  carbon  dioxid  and  water;  the  rest  of  the  plant  all  the  time 
and  the  whole  of  the  plant  at  night  must  have  organic  food  like  all  other 
living  things. 

Antithesis  avoidable.  — A  little  consideration  shows  that  the  apparent 
antithesis  between  green  plants  and  other  creatures  is  of  our  own  making; 
it  is  produced  solely  by  the  application  of  the  term  food  to  the  sub- 
stance- which  enter  the  body,  irrespective  of  their  role.  This  antithesis 
can  be  avoided,  and  the  confusion  and  contradiction  eliminated,  merely 
by  avoiding  this  inept  use  of  the  term  food  and  by  applying  it  to  organic 
substances  only.  By  this  expedient  we  es<  ape  a  different  use  of  the  same 
terms  in  plant  and  animal  physiology,  with  its  resultant  confusion  of 
idea-,  and  we  bring  the  green  plant-  into  line  with  all  other  beings,  SO 
far  as  nutrition  is  concerned.  Excluding  the  inorganic  substances  from 
the  category  of  foods,  we  need  to  recognize  thai  one  power  possessed  by 
green  plant-  is  unique:  they  alone  make  their  own  food,  and  not  their 
own  only,  but  food  for  the  whole  world.  What  they  use  for  this  i><>"\ 
making  —  carbon  dioxid  and  water— may  be  distinguished  as  food  ma- 


358  PHYSIOLOGY 

terials.  What  they  make  is  universally  known  as  food  for  their  colorless 
cells,  for  non-green  plants,  and  for  animals.  Why  should  it  not  also  be 
recognized  as  their  own  food? 

Food  for  plants  is  organic.  —  Food  for  plants,  then,  is  like  food  for 
animals,  always  organic,  the  product  of  living  beings;  and  in  the  last 
analysis,  all  food  is  made  by  green  plants,  for  they  alone  among  living 
beings  have  the  power  of  making  it  out  of  the  simple  compounds  C02 
and  FLO.1  They  make  it  only  in  the  green  cells  by  the  aid  of  light; 
and  they  make  so  much  that  they  feed  not  only  themselves,  but  all  other 
creatures.  The  lion  may  live  exclusively  on  flesh,  but  the  flesh  was 
built  up  by  the  herbivorous  animal  from  the  herbage  it  grazed,  and  the 
herbage  was  nourished  by  the  foods  it  could  itself  make  in  sunlight. 
Man  grows  plants  and  appropriates  the  leaves,  the  roots,  the  stems,  the 
fruits,  or  the  seeds,  improved  by  his  selection  and  loaded  with  surplus 
food,  for  his  own  nourishment;  or  he  feeds  the  steer,  the  sheep,  and  the 
hog  with  grass  or  grain  that  he  may  later  use  their  flesh  for  his  food. 

What  are  the  plant  foods  ?  —  Having  established  a  general  meaning  for 
the  word  food,  the  next  question  is:  To  what  specific  substances  is  it 
to  be  applied?  Foods  come  from  many  sources  and  are  of  many  kinds; 
and  because  they  are  so  various,  only  the  principal  classes  can  be  named, 
and  a  few  examples  briefly  described.  The  four  most  important  sorts 
are  carbohydrates,  fats,  amides,  and  proteins. 

Carbohydrates.  —  Some  carbohydrates  are  directly  made  by  green 
plants;  but  there  are  also  many  that  are  secondary  products.  The 
name  is  no  longer  used  in  chemical  classification;  it  is  rather  convenient 
than  exact,  just  as  "  cryptogam  "  among  plants  or  "  invertebrate  " 
among  animals.  Here  belong  the  sugars,  the  starches,  and  the  celluloses, 
each  probably  comprehending  an  indefinite  number  of  different  in- 
dividuals. This  is  certain  among  the  simpler  sugars,  whose  composi- 
tion is  known;  but  only  hypothetical  ft  starch  and  cellulose,  whose 
complexity  has  hitherto  baffled  all  analysis. 

All  these  substances  have  a  composition  like  this  :  C„H2»On,  or  CnH2(n-i)0(„_i), 
or  C„H2(n_2)  0(n-2),  in  which  the  value  of  n  is  6  or  a  multiple  of  6  where  known, 
but  may  run  up  to  several  hundred.  Thus  grape  sugar  and  its  allied  hexoses  all 
contain  C6H12O6;  while  cane  sugar  and  its  allies  consist  of  C12H22O11.  Starch  and 
cellulose  can  be  represented  only  as  n  (CeHioOs),  with  the  value  of  n  quite  uncertain, 
but  large.    These  empirical  formulas,   however,  cannot  convey  any  idea  of  the 

1  Certain  bacteria  also  seem  to  be  able  to  utilize  these  substances  to  form  foods;  but 
so  far  as  is  known  the  product  is  utterly  trivial  in  amount,  and  the  fact  is  entirely  without 
significance,  were  it  not  for  its  exceptional  character. 


NUTRITION  359 

complexity  of  even  the  amplest  i  arbohydrates,  aor  of  the  fact  that  a  mere  difference 
in  the  position  of  certain  atoms  or  groups  of  atoms,  whi<  li  does  qoI  affet  I  the  per- 
centage composition  at  all,  gives  «  holly  different  <  hemi<  al  and  physic  al  i  haracters 
to  the  substance 

Thus,  grape  sugar  (glucose)  exists  in  two  forms,  one  of  which  rotates  a  beam  of 
polarized  light  to  the  right  and  the  other  to  the  left;  the  one,  i-glu<  ose,  is  abundant 
in  plants;  the  other,  /-glucose,  does  not  o<  i  ur  in  nature  but  has  been  made  art i- 
ficially.  The  difference  is  shown  partly  in  the  three  following  structural  formulas, 
which  all  sum  up  (V,l  I  pj<  »« : 

OH     H     OH  OH 

!      I      I      I 

COH — C C — C — C CH2OH       =<*-glucose 

I         I         I         I 
H    OH    H      H 

H    OH     H      H 

I       !       I       I 

COH — C — C — C C CH2OH       =/-glucose 

I         I         I         I 
OH    H    OH  OH 

Further,  fruit  sugar  ((/-fructose)  is  abundant  in  plants,  and  its  structure  is  quite 

different  from  glucose: 

H     OH  OH 

I         I         I 
CHoOH — CO — C — C — C — CHoOH     =<Z-fructose 

I         I         I 
OH     H       H 

Another  sugar  especially  abundant  in  plants,  cane  sugar,  C12H22O11,  probably  has 
this  formula: 


saccharose 

and  when  it  breaks  at  the — O —  bond,  it  takes  up  If  •<  >H  and  resolves  itself  tntoa 
molei  ule  of  glui  ose  and  a  mole*  ule  of  fru<  lose.  These  two  hexose  sugars,  glucose 
and  fructose,  and  the  disaccharide,  cane  sugar,  are  the  only  sugars  which  occur  in 
abundance  in  plants;  though  mannose,  galactose,  and  maltose  are  formed  in  the 

( ..u isc>  of  digestion. 


360  PHYSIOLOGY 

The  simplest  carbohydrate  which  has  been  detected  in  plants  is 
formaldehyde,  HCOH.  This  group  will  be  recognized  in  the  makeup  of 
all  the  more  complex  ones  above  (but  see  p.  375).  While  it  has  only  a 
transient  existence  and  does  not  occur  free,  except  in  minute  amounts, 
it  has  its  special  significance  in  that  it  is  probably  the  first  substance 
formed  by  the  green  cells  from  water  and  carbon  dioxid. 

Fats.  —  Fats  are  apparently  always  secondary  products,  and  consti- 
tute a  common  form  of  surplus  food.  These  storage  products  furnish 
various  commercial  oils;  e.g.  olive  oil,  cotton  oil,  linseed  oil,  castor  oil, 
corn  oil,  etc.  They  occur  usually  in  fluid  form  as  minute  droplets  in 
the  protoplast,  only  occasionally  being  solid  at  ordinary  temperatures, 
as  in  the  seed  of  cacao.  They  are  of  very  complex  structure,  being  com- 
pounds of  glycerin  and  three  molecules  of  fatty  acid. 

Their  structure  may  be  understood  from  these  formulas: 

CH2OH  CH2  •  R 

I  I 

Glycerin  is:  CHOI  I  A  fat  is:  CH  •  R 

I  I 

CH2OH  CH2  •  R 

in  which  R  may  represent  oleic  acid  (CisH3402),  linoleic  acid  (Ci8H3202),  hypogaeic 
acid  (Ci6H3o02),  or  any  other  member  of  a  considerable  series  of  fatty  acids,  minus 
the  acid  ion  H.  The  R  radicals  may  be  all  alike  or  different.  When  digested,  fats 
break  up  into  glycerin  and  the  fatty  acid  or  acids.  The  fats  contain  a  notably 
small  proportion  of  oxygen. 

The  lecithins  are  substances  allied  to  the  fats  in  their  constitution, 
containing  phosphoric  acid  and  cholin  in  place  of  one  of  the  fatty  acid 
radicals,  R.  They  are  very  widely  distributed  in  plants,  and  probably 
play  an  important  role  in  the  protoplasm,  but  just  what  is  not  known  at 
present.  It  may  be  that  they  determine  what  substances  may  pass 
through  the  membranes;  and  it  may  be  also  that  they  are  connected 
with  the  formation  of  chlorophyll. 

Amides.  — The  name  is  here  used  loosely  and  not  in  its  strict  chemical 
sense,  for  a  group  of  substances  of  which  none  are  popularly  known. 
For  convenience,  they  may  be  distinguished  as  nitrogenous  compounds 
intermediate  between  carbohydrates  and  proteins.  On  the  one  hand, 
they  are  derivatives  of  proteins,  among  whose  decomposition  products 
various  amino-acids  always  figure.  On  the  other  hand,  they  are  deriv- 
atives of  the  carbohydrates  and  their  allies,  from  which,  with  proper 
additions,  they  are  readily  formed.  In  addition  to  the  carbon,  hydrogen , 
and  oxygen  of  carbohydrates,  they  contain  nitrogen,  always  combined 


NUTRITION  36] 

with  hydrogen  as  a  definite  radical,  XII,,  known  as  the  amide  radical. 
It  may  replace  an  II  « >r  < ) 1 1  group  in  the  various  carbohydrates  and  their 
allied  acids,  converting  them  by  this  slight  change  into  quite  different 
substances. 

Thus,  either  aceti<  acid,  CHa  COOH,  or  glycolic  acid,  CH9OH  COOH,  be- 
comes amido-acetic  acid  (glycin),  CHa(NH2)  COOH,  by  the  substitution  of  the 
amide  radical  Nil-  for  hydrogen  (II)  or  hydroxy!  (OH),  respectively.  Glucose, 
(Mil     ciloll     CHOH     CHOH     CHOH— CH2OH,  becomes  glucosamin  COH 

CIIiNHj)— CIIOII— CIIOII— CHOH— CII.jOH,  by  a  like  substitution.  On  the 
other  hand,  some  of  the  constant  decomposition  products  of  the  more  complex 

CH3X 
proteins  are  glycin  (ante);  leucin,  yCH.—CHz—CH.(Nlh)—<:OOU;  tyrosin, 

CH3/ 

HC— CH 
OH— c/         V;— CH2— CH(Nh2)— COOH;  asparagin,   CO(NH2)— CH(NHS) 

HC=CH 
— CHj — COOH;  in  all  of  whi<  h  the  amide  radical  has  replaced  II  orOII  of  an  allied 
substance. 

Proteins.  —  Proteins  are  the  substances  which  compose  the  larger  part 
of  the  cytoplasm;  protein  foods,  therefore,  are  those  which  can  be  most 
directly  used  for  nourishment,  and  so  represent  the  end  of  food  making. 
To  define  proteins  is  quite  impossible;  they  are  so  numerous  and  so 
varied  that  scarcely  any  characteristic  is  universal.  Within  this  huge 
group  are  included  some  substances  which  are  relatively  simple,  and 
others  whose  complexity  defies  all  analysis.  Even  the  simplest  are 
scarcely  known  chemically,  the  actual  knowledge  permitting  only  theo- 
ries of  their  construction.  It  has  been  possible  in  most  cases  to  deter- 
mine only  the  percentage  composition,  which  with  a  study  of  the  decom- 
position products  sometimes  permits  the  establishment  of  an  empirical 
formula.  The  more  complex  proteins  contain  sulfur,  and  some  have 
also  phosphorus  in  addition  to  the  carbon,  hydrogen,  oxygen,  and 
nitrogen  of  amides,  with  traces  of  ash,  which  may  or  may  not  be  struc- 
turally a  part  of  the  protein.  One  nearly  pure  protein  is  familiarly 
known,  the  albumin  or  "  white  "  of  eggs;  perhaps  the  best  known  plant 
protein  is  the  one  longest  known,  the  gluten  of  wheat  grains. 

To  illustrate  the  complexity  of  these  substam  es  and,  as  well,  the  uncertainty  re- 
garding their  composition,  the  following  formulas,  though  hardly  more  than  guesses, 

are  quoted.  A  crystalline  vitelfin  from  squash:  C jgd  1  ( _,\ ■.,.,<  >s.s..  ^"  albumin: 
CnoHiiMNgMOMsSs.     Hemoglobin    (of   the   blood):    CnaHiwoNsiiOMjFeSa;    the 

same,  another  guess,  C6,„,l  [geoN  ui1  'l7»FeS|. 


362  PHYSIOLOGY 

The  most  familiar  physical  characteristic  of  many  proteins  is  that  they 
coagulate;  heat,  prolonged  shaking,  the  action  of  acids,  alcohol,  salts, 
etc.,  cause  the  protein  to  change  from  a  liquid  or  semi-liquid  form  to 
a  firmer  "  clot,"  which  by  pressure  can  be  separated  into  a  fluid  and  a 
more  solid  portion.  The  coagulation  of  white  of  egg  by  heat,  of  milk  on 
souring,  and  of  the  fibrin  of  blood  on  contact  with  a  vessel  are  familiar 
examples.  Ordinarily  the  coagulum  is  insoluble  in  water.  But  the  neu- 
tral salts  act  differently,  producing  a  soluble  clot.  Advantage  is  taken 
of  this  fact  to  separate  various  mixed  proteins  and  purify  them  partially 
for  analysis  by  "  salting  out."  Other  physical  peculiarities  are  their 
high  resistance  to  the  electric  current,  their  large  molecular  weight  (prob- 
ably 15,000  and  more  in  many  cases)  and  hence  slow  diffusibility,  so 
slow  usually  as  to  be  negligible. 

Some  proteins  crystallize,  but  most  do  not.  When  first  discovered 
such  crystals  were  called  "  crystalloids,"  because  it  was  not  believed 
that  true  crystals  could  be  formed  by  organic  matter.  They  are  regularly 
present  in  the  protein  grains  of  the  Brazil  nut,  castor  bean,  etc.  (fig.  664). 

Plant  foods  again.  —  Plant  foods,  then,  are  specifically  these  complex 
organic  compounds  —  not  the  simple  inorganic  substances  out  of  which 
green  plants  alone  can  make  food.  This  is  practically  implied  in  the 
terms  proposed  by  authors  who  reject  this  use  of  the  term  food,  and 
used  frequently  to  distinguish  plants  as  to  their  mode  of  nutrition,  viz. 
autotrophic,  or  self-nourishing,  plants,  and  heterotrophic  plants.  The  ob- 
vious objection  to  these  two  terms,  if  they  are  anything  more  than  con- 
venient and  figurative  ones,  is  that  only  some  parts  of  most  so-called 
autotrophic  plants  are  strictly  self-nourishing.  Only  the  plants  whose 
every  cell  contains  chlorophyll  are  actually  autotrophic.  If  the  term 
be  used  in  the  wide  sense,  green  plants  are  not  merely  self-nourishing 
—  they  nourish  all  living  things. - 

Kinds  of  food  needed.  —  However,  there  is  a  wide  difference  among 
plants  as  to  the  kind  of  food  that  they  require.  The  known  variety  is 
so  great  that  it  is  impracticable  to  state  it  in  detail  here,  and  only  a 
small  number  of  plants,  chiefly  fungi,  have  been  carefully  studied  in  this 
respect.  Some  thrive  best  on  comparatively  simple  compounds;  others 
require  the  most  complex  proteins.  Some  flourish  on  material  which  is 
useless  or  even  highly  injurious  to  others.  The  proverb  "  what  is  one 
man's  meat  is  another  man's  poison,"  is  quite  applicable  to  plants. 
Among  the  lowest  and  simplest  plants,  the  bacteria,  there  are  some  which 
live  upon  substances  almost  as  simple  as  the  food  materials  of  higher 


NUTRITION  363 

plants;  bul  they  manage  to  secure  energy  in  ways  unknown  to  us,  and 
build  these  substances  into  their  bodies. 

Food  a  source  of  energy.  —After  all,  foods  arc  of  value  to  plants,  as 
we  conceive  things,  because  they  supply  them  with  energy  as  well  as 
with  material.  The  energy  income  in  this  way  is  indeed  the  important 
feature.  The  green  plant  locks  up  in  the  food  it  constructs  a  fraction 
of  the  solar  energy  which  reached  it  as  light;  and  thus  this  energy 
becomes  available  to  other  organisms,  since  after  further  transforma- 
tions of  the  foods  they  can  release  it  by  decomposition  and  apply  it  to 
other  reactions. 

Food  and  growth.  —  Because  with  our  best  appliances  we  are  unable 
to  know  yet  the  real  nature  of  nutrition,  the  use  which  a  plant  makes  of 
food  can  be  determined  only  by  the  extent  to  which  it  promotes  growth 
and  development  of  the  body.  The  term  economic  coefficient  has  been 
used  to  express  the  ratio  which  the  increase  in  the  weight  of  a  crop  (say 
of  a  fungus)  bears  to  a  given  quantity  of  a  particular  food.  Manifestly 
there  are  other  ways  in  which  the  plant  uses  a  food  besides  incorporating 
it  into  the  permanent  structure  of  the  body,  and  many  complicated  rela- 
tions may  be  disturbed  by  too  limited  nutrition.  Yet  this  economic  co- 
cfli.  ient  expresses,  in  a  crude  way,  die  differences  in  the  availability  of 
foods  for  body  building,  and  so  impresses  the  fact  that  the  processes 
of  nutrition  differ  widely  in  different  plants. 

2.    PHOTOSYNTHESIS 

The  fundamental  fact  in  the  nutrition  of  all  living  things  is  the  capacity 
of  green  plants  to  mnke  certain  complex  organic  compounds,  carbohy- 
drates namely,  out  of  carbon  dioxid  and  water,  by  the  aid  of  light.  This 
unique  process  is  known  as  photosynthesis. 

The  term  used.  —  When  the  food  of  green  plants  was  described  as 
inorganic,  this  transformation  of  inorganic-  materials  into  carbohydrates, 
which  was  taken  to  be  their  incorporation  into  the  body,  was  called  assimi- 
lation, after  the  analogy  of  the  transformations  undergone  by  the  food 
of  animals.  As  the  radical  differences  between  the  food  making  of  a  green 
plant  and  true  assimilation  in  both  plants  and  animals  began  to  appear, 
an  attempt  was  made  to  obviate  the  confusion  by  using  the  term  carbon 
assimilation.  These  terms  are  still  in  common  use  in  other  countries, 
but  will  gradually  disappear.1    Clearness  demands  the  use  of  the  (lis- 

1  For  sximple,  a  recent  hybrid  is  "photosynthi  1  i.  carbon   1    imitation"' 


364  PHYSIOLOGY 

tinctive  term  photosynthesis  for  the  process  that  is  peculiar  to  green 
plants,  leaving  the  term  assimilation  to  be  applied  to  the  same  process 
in  both  plants  and  animals;  namely,  to  the  transformation  of  foods  of  all 
kinds  into  the  actual  living  stuff. 

As  photosynthesis  requires  a  supply  of  certain  substances,  which  re- 
appear in  more  elaborate  form,  and  acts  through  certain  structures,  which 
require  a  supply  of  energy  for  doing  the  work,  the  making  of  carbohy- 
drates may  be  described  appropriately  in  terms  of  a  manufacturing  pro- 
cess. There  are  (1)  the  raw  materials,  (2)  the  laboratories,  (3)  the  en- 
ergy, (4)  the  products  and  the  process. 

(1)    The  Raw  Materials 

Carbon  dioxid. — The  raw  materials  needed  have  already  been 
named,  carbon  dioxid  and  water.  Carbon  dioxid  exists  everywhere  in 
the  air,  in  the  ratio  of  about  3  parts  in  10,000,  and  its  nearly  uniform 
distribution  is  assured  by  the  convection  currents  (winds)  that  stir  the 
atmosphere.  Only  in  the  neighborhood  of  cities  or  other  places  where 
C02  is  being  produced  in  quantity  is  there  temporarily  an  excess.  By 
decomposition  of  rocks,  burning  of  fuel,  decay  of  organic  matter,  and 
respiration  of  plants  and  animals,  the  supply  of  C02  is  maintained,  though 
great  quantities  are  removed  from  the  air  by  green  plants.  The  amount 
is  constant,  so  far  as  can  be  known  historically,  though  there  is  geological 
evidence  that  in  earlier  periods  of  the  earth's  development  COs  existed 
in  much  larger  and  also  in  smaller  quantities  than  now,  since  enormous 
amounts  have  been  fixed  in  beds  of  limestone,  and  later  released  by 
weathering. 

C02  near  the  ground. — On  quiet  days  there  is  a  layer  of  air  near 
the  ground  where  the  proportion  may  rise  much  higher  (10  to  12 
times  as  much),  owing  to  the  diffusion  of  C02  from  the  soil,  where 
it  is  being  evolved  by  the  decomposition  of  organic  matter  through 
the  agency  of  bacteria,  etc.  Perhaps  turf- forming  and  rosette  plants 
profit  from  the  lowly  position  of  their  leaves,  since  the  more  C02  in 
the  air,  within  limits,  the  more  can  enter  them  and  be  used  for  food 
making. 

CO,  in  water.  —  In  the  water  of  quiet  pools  and  lakes,  as  well  as  in 
slow  streams,  the  amount  of  C02  dissolved  is  much  greater  than  in  the 
air.  It  is  produced  by  the  host  of  organisms  living  in  the  waters  and 
by  decay,  and  is  also  dissolved  from  the  air.  As  C02  is  very  soluble 
in  water  (up  to  volume  for  volume  at  ordinary  temperatures),  it  may 


NUTRITION  365 

thus  accumulate  to  25  or  even  100  times  as  much  as  in  the  air.    This 
puts  water  plains  in  a  very  advantageous  position  so  far  as  a  supply  of 

( -( ).  is  1  "iii  erned. 

Admission  of  COo. — Of  course  in  all  plants  that  present  an  uncutin- 
ized  (ami  consequently  a  wet)  surfa<  e  t<>  the  air,  the  ('<  >_.  enter-  direi  th- 
at the  surface;  in  fact  it  can  enter,  in  proportion,  wherever  water 
can  evaporate.  As  the  cuticular  evaporation  in  most  of  the  higher 
plants  is  small,  the  quantity  of  C02  entering  through  the  epidermis 
is  trifling.  Into  some  epiphytic  seed  plants  which  have  no  stomata 
(e.g.  Tillamhia),  the  leaves  of  mosses,  the  thallus  of  liverworts,  etc.,  C02 
enters  directly. 

The  supply  for  the  great  majority  of  the  larger  land  plants,  however, 
passes  through  the  stomata.  These  openings  are  ample  to  admit  not 
only  what  is  net  essary,  but  five  or  six  times  more  than  actually  passes 
through  them  in  nature. 

It  has  been  shown  that  C02  will  diffuse  through  a  multiperforate  partition, 
placed  over  some  ready  solvent  like  sodium  hydroxid,  as  freely  as  it  would  enter 
the  solvent  were  the  partition  absent,  provided  the  perforations  are  not  farther  apart 
than  ten  times  their  diameter.  The  epidermis  is  like  such  a  multiperforate  parti- 
tion in  which  the  area  of  the  openings  is  scarcely  more  than  I  per  cent  of  the  total 
surface.  Hut  the  CO2  dissolves  so  readily  in  the  wet  cell  walls  bounding  the  inter- 
cellular spaces  that  its  pressure  in  the  internal  passages  is  usually  o;  so  it  may 
traverse  the  stomata  as  rapidly  as  is  permitted  by  the  gradient  of  pressure,  0.228  mm. 
outside  to  o  inside.  The  speed  of  the  molecules  is  found  to  he  greatly  accelerated 
as  they  swirl  through  the  narrow  passage  of  a  stoma;  in  fact,  they  traverse  it  at  a 
speed  about  50  times  as  great  as  when  diffusing  freely  into  sodium  hydroxid. 

Even  when  the  orifice  of  the  stoma  is  partly  closed,  though  this  reduces 
proportionally  the  amount  of  gas  passing,  the  supply  of  C02  is  not  likely 
to  fall  below  the  maximum  that  can  be  used.  As  in  good  light  the  sto- 
mata are  usually  more  than  half  opened,  even  though  the  evaporation 
i-  excessive,  an  adequate  supply  of  C02  is  thus  assured,  so  far  as  admis- 
sion to  the  aerating  system  is  concerned. 

Deficiency  in  C0L..  A-  a  matter  of  fact,  however,  the  supply  of  O  », 
is  often  less  than  muld  be  utilized  by  the  chloroplasts.  This  is  shown 
by  the  fact  that  photosynthesis  is  increased  when,  in  good  light,  the 
amount  of  COs  in  the  air  around  tin-  plant  is  artificially  increased.  The 
increase  may  go  to  a  hundredfold  or  more  with  positive  benefit,  at  least 
so  far  as  brief  experiments  -how.  Any  increase  in  the  air  mean-  in- 
creased pressure  of  COs  in  the  aerating  passages;  and  this  mean-  the 
solution  of  more  C(  >..  in  the  wet  walls,  and  consequently  faster  diffusion 


366  PHYSIOLOGY 

toward  the  chloroplasts,  where  the  COa  is  actually  utilized.  Here, 
indeed,  is  the  point  at  which  the  normal  pressure  of  C02  usually  limits 
the  process  of  photosynthesis.  The  main-line  transportation  through 
stomata  and  intercellular  spaces  is  adequate,  but  the  switching  facilities 
in  the  terminal  yards  (from  cell  wall  to  chloroplast)  are  not;  hence  when 
otherwise  capable  of  operating  to  full  capacity,  the  laboratories  are 
hindered  by  the  impossibility  of  securing  enough  of  this  raw  material. 
There  are  other  factors  which  may  limit  the  output,  to  be  discussed 
later;  but  the  shortage  of  C02  due  to  low  diffusion  pressure  is  the  com- 
monest. 

Water.  —  Water,  the  other  of  the  raw  materials,  is  never  lacking  when 
plants  are  active.  Its  source  for  most  land  plants  is  the  soil  water  that 
enters  through  the  roots.  The  little  that  may  enter  via  the  leaves  (com- 
parable with  the  amount  leaving  in  the  same  time  by  cuticular  evapora- 
tion, p.  327)  is  practically  negligible.  Only  in  mosses,  liverworts,  and 
a  few  epiphytes,  i.e.  plants  with  practically  uncutinized  surfaces,  may 
it  freely  enter  aerial  parts.  In  many  such  cases  there  are  special  struc- 
tures that  hold  water  until  it  can  enter. 

Relation  of  C02  and  H20.  — The  carbon  dioxid  and  water  enter  into 
a  double  relation.  In  part,  the  C02  is  merely  dissolved  in  the  water; 
in  part  the  two  form  a  loose  chemical  combination,  carbonic  acid,  H2C03. 
This  three-phase  system,  solute,  solvent,  compound,  is  in  equilibrium, 
and  if  the  amount  of  any  member  is  altered,  corresponding  changes  take 
place  in  others  and  equilibrium  is  again  reached. 

(2)   The   Laboratories 

Chloroplasts.  — The  laboratories  in  which  photosynthesis  proceeds 
are  the  chloroplasts.  These  are  organs  of  various  form  and  size,  found 
only  in  superficial  parenchyma  cells,  chlorenchyma,  of  stems  and  foliage. 
(For  a  discussion  of  this  tissue  and  its  relations  to  external  agents, 
see  Part  III,  p.  530.)  The  chloroplasts  are  embedded  in  the  cytoplasm 
just  within  the  ectoplast  and  marked  by  their  green  color.  In  a  few  algae 
(especially  the  Conjugates,  p.  37)  they  have  various  and  sometimes 
fantastic  forms,  but  in  almost  all  the  higher  plants  they  are  shaped  like 
a  bun  or  a  thick  round  cake;  that  is,  two  diameters  are  nearly  equal, 
and  the  other  is  shorter,  with  the  convexity  greater  on  one  face  than  the 
other  (see  fig.  619,  p.  297).  Their  form  is  subject  to  change  from  internal 
causes,  and  in  moving  about  with  the  cytoplasm  they  are  easily  distorted 


NUTRITION  367 

by  pressure,  showing  thai  they  arc  of  a  soft,  elastic,  and  semi-fluid  con- 
sistent v. 

Pigment  and  stroma.  —  In  fact,  the  body  or  stroma  of  the  chloroplasts 
seems  to  be  like  the  cytoplasm,  but  dyed  by  the  green  pigment.  The 
precise  relation  between  the  pigment  and  the  stroma  lias  not  been  satis- 
factorily made  out,  even  in  the  killed  chloroplast,  and  in  the  live  un- 
altered chloroplasts  it  can  only  be  conjectured.  In  some  cases,  when 
the  pigment  has  been  dissolved  out  by  alcohol,  the  stroma  (of  course 
coagulated  by  the  alcohol)  presents  a  spongy  appearance,  and  it  has 
been  inferred  that  the  meshes  of  the  sponge  throughout  were  occupied  by 
pigment.  In  others,  especially  in  the  larger  chloroplasts  which  can  be 
sectioned,  the  pigment  seems  to  be  restricted  to  a  spongy  shell  of  measur- 
able  thickness  at  the  surface,  while  the  interior  is  colorless. 

Pigments. — The  yellow-green  pigment  is  tailed  chlorophyll;  but 
it  is  not  a  single  substance.  Several  pigments  can  be  separated  more  or 
less  completely,  of  which  only  two  are  abundant  and  constant  in  all  higher 
plants,  the  one  bluish  green  and  the  other  pale  yellow.  The  names 
applied  to  these  are  confusing.  To  distinguish  them  we  shall  employ 
the  terms  chlorophyllin  and  carotin.  To  the  bluish  green  one  no  dis- 
tinctive term  has  been  generally  applied,  but  it  has  been  usually  called 
chlorophyll  (not  distinguishing  it  from  the  combination),  or  chlorophyll 
proper.  For  the  yellowish  one,  xanthophyll,  etiolin,  and  carotin  have 
been  used.    The  last  is  preferable. 

The  term  xanthophyll  is  descriptive,  but  it  has  also  been  used  for  other  minor 
yellow  pigments.  Etiolin  was  applied  to  tin-  pale  yellow  pigment  which  appears 
when  plants  have  been  "etiolated"  by  being  grown  or  kept  for  a  time  in  darkness. 

It  seems  to  be  identical  with  the  yellow  pigment  named  from  the  carrot,  carotin, 
which  proves  to  be  very  widely  distributed  in  plants. 

Chlorophyllin  and  carotin  may  be  partially  separated  by  their  unequal 
solubilities.  If  to  a  fresh  solution  of  chlorophyll  in  80  per  cent  alcohol, 
benzene  be  added,  the  mixture  shaken,  and  then  allowed  to  stand,  the  ben- 
zene ri>es,  carrying  the  greater  part  of  the  chlorophyllin,  while  the  alco- 
hol retains  the  greater  part  of  the  carotin. 

Chlorophyllin.  The  chemical  composition  of  chlorophyllin  is  not 
known.  It  is  very  easily  altered  ami  i-  certainly  very  complex,  contain- 
ing N  as  well  a-  C,  H,  and  ( ).  Whether  phosphorus  or  magnesium  is 
an  essential  constituent  is  in  contention.  Iron  does  not  seem  to  be  an 
integral  part  of  it,  though  considered  essential  to  it-  formation.  The 
red  coloring  matter  of  the  blood,  hemoglobin,  yield-  do  omposition  prod- 


368  PHYSIOLOGY 

ucts  very  like  those  of  chlorophyllin,  suggesting  that  the  two  pigments 
have  structural  similarities.  That  both  have  peculiar  relations  with 
carbon  dioxid  is  interesting,  but  cannot  yet  be  explained. 

When  chlorophyllin  disappears  in  the  autumn,  the  yellow  pigments  become 
prominent,  and  some  of  its  decomposition  products  have  a  share  in  reddening  the 
tissues.  The  red  pigments  are  then  dissolved  in  the  cell  sap ;  the  yellows  are  still 
in  the  chloroplasts.     The  autumnal  coloring,  however,  is  not  yet  fully  understood. 

Carotin. — The  chemical  composition  of  carotin  is  certainly  very 
different  from  that  of  chlorophyllin.  Its  formula,  probably  C^H^  or 
C4oH56,  shows  that  it  lacks  both  O  and  N.  It  is  widely  distributed  in 
plants,  and  to  it  chiefly  the  orange  and  yellow  tints  of  flowers,  fruits, 
seeds,  roots,  etc.,  are  due. 

(3)   The  Energy 

Light.  —  While  the  intricate  chemical  relations  of  chlorophyll  are 
yet  unknown,  one  of  its  physical  features  is  known  to  be  of  the  greatest 
importance.  That  is  its  capacity  to  absorb  radiant  energy.  When  the 
radiant  energy  coming  from  the  sun  is  passed  through  prisms  of  rock 
salt,  glass,  or  other  appropriate  media,  or  is  reflected  from  a  minutely 
striate  surface,  the  various  wave  lengths  are  unequally  refracted  or 
reflected,  so  that  the  physiological  and  other  effects  of  energy  of  dif- 
ferent wave  lengths  can  be  studied.  Certain  of  these  wave  lengths 
(if  they  were  sound  waves  one  might  say  about  1  octave  out  of  11)  affect 
our  eyes,  and  this  physiological  effect  is  what  we  know  as  light.  By 
a  figure  of  speech  the  cause  is  likewise  so  named,  and  the  waves  them- 
selves are  called  "  light."  But  they  differ  only  in  length  and  frequency 
from  the  much  greater  number,  both  longer  and  shorter,  slower  and 
faster,  which  we  cannot  perceive  with  our  eyes.  Other  physiological 
effects,  such  as  inflammation  of  the  skin  and  the  development  of  pig- 
ment ("sunburn  "),  are  produced  by  light  waves.  On  the  plant,  like- 
wise, waves  of  different  lengths  produce  different  effects  according  as 
certain  parts  are  attuned  to  them  (see  p.  449). 

Absorption  spectrum.  — The  chlorophyll  is  so  constituted  that  it  can 
absorb  waves  of  certain  lengths,  all  falling  within  the  range  of  our  vi- 
sion. On  the  plant  this  energy  cannot  produce  the  effect  that  it  does  on 
our  eyes,  and  hence  for  the  plant  it  is  "  light  "  only  by  a  convenient 
figure  of  speech.  There  are  seven  separated  groups  of  waves  whose 
absorption  is  more  or  less  complete.     When  we  look  at  a  spectrum  of 


NUTRITION 


369 


sunlight,  i.e.  a  narrow  liar  of  light  dispersed  into  a  band  of  different 
wave  lengths,  each  group  of  waves  produces  its  appropriate  effeel  and 
we  see  a  band  of  blending  colors,  'lark  red  at  one  end,  running  through 
ted,  orange,  yellow,  green,  blue,  indigo,  violet,  and  ending  in  the  dark- 
est violet.  On  interposing  a  leaf  in  the  path  of  the  light,  there  appear 
across  the  spectrum  dark  strips  due  to  the  partial  or  complete  stoppage 
of  the  energy.  Similar  absorption  hands,  slightly  displaced,  are  seen  by 
using  in  the  same  way  an  alcoholic  solution  of  chlorophyll  (tig.  648). 

a   B   C  D  E  b  F  G  h 


Fig.  648.  —  Absorption  spectra:  A,  chlorophyll  of  Allium  ursittum  in  alcohol;  B, 
chlorophyll  of  English  ivy  (II edera  Helix)  in  alcohol;  C,  chlorophyll  of  Oscili 
alcohol;  D,  carotin,  i,  2,  3,  4,  absorption  bands  of  chlorophyllin;  /,  //,  ///,  absorp- 
tion bands  of  carotin;  EA,  end  absorption.  The  Intend  broken  lines  mark  tin-  position 
of  the  ['rincipal  absorption  lines  of  the  solar  spectrum  (Fraunhofer  lines);  the  numbered 
solid  lines  form  a  si  ale  from  which  wave  lengths  (\)  in  millionths  of  a  millimeter  may  be 
found  by  adding  a  cipher;  note  the  increasing  dispersion  from  left  (red)  to  right  (violet). 
—  After  Koiil. 

These  absorption  bands  arc  as  follows:  1,  in  the  red  a  wide  black  "tie.  its  wave 
lengths  i\i  being  67c— 635  nn'  ;  2,  a  narrower  and  less  intense  one  in  the  orange, 
X=622-5o;  mm;  3,  in  the  yellow,  a  band  much  lighter  than  a,  and  shading 
out  on  the  sides,  \  ^S;-^,^  uix;  (,  a  faint  band  in  the  green,  not  always 
to  be  seen,  and  probably  due  to  decomposition  products,  X  =  544-5,^0  mm-  ( >r- 
dinarily  the  other  three  blend  into  one,  and  there  are  no  visible  waves  left  beyond 
the  blue  (X= 495-420).  By  very  careful  manipulation,  using  dilute  solutions  in- 
stead of  a  leaf,  they  .an  be  distinguished,  their  limits  not  being  sharply  marked. 
1  The  exact  location  of  the  bands  varies.  1  /xfi  —  0.000001  nun. 
C.  B.  .v  C.  no  1  \\y        j  1 


370  PHYSIOLOGY 

The  bands  1-3,  and  possibly  4,  belong  to  chloropbyllin,  while  the  indefinite  three, 
I-III,  belong  to  carotin.  These  three  are  much  better  seen  in  the  absorption 
spectrum  of  carotin  alone  (fig.  648,  D). 

Fluorescence.  — Chlorophyll  has  another  physical  character,  which  it  shares  with 
some  other  dyes ;  its  solution  is  fluorescent.  When  a  strong  solution  in  alcohol  is 
held  between  the  eye  and  the  light,  the  color  is  a  vivid  green;  but  if  examined  by 
bright  reflected  light,  it  appears  deep  blood-red.  While  this  is  a  useful  recognition 
mark,  the  physiological  significance  of  fluorescence,  if  any,  cannot  be  explained. 

The  absorbed  energy.  — The  energy  that  drives  the  machinery  is  de- 
rived from  light,  for  if  a  green  plant  be  kept  in  darkness,  it  is  entirely 
unable  to  make  any  carbohydrates.  Furthermore,  it  is  only  the  chloro- 
plast  directly  illuminated  that  receives  this  energy.  A  lighted  portion 
of  a  leaf  cannot  communicate  the  energy  to  a  darkened  area.  If  a  por- 
tion of  a  leaf  be  covered  with  an  opaque  plate,  while  C02  is  allowed  free 
access,  the  rest  of  the  leaf  may  show  evidence  of  active  photosynthesis, 
but  the  darkened  area  shows  none.  Moreover,  it  is  the  energy  absorbed 
by  the  chlorophyll  that  does  the  work. 

The  following  experiment  shows  this:  A  plant  was  kept  in  the  dark  until  its 
leaves  showed  no  trace  of  starch.  Then  on  a  sunny  day  a  spectrum  of  sunlight,  as 
bright  as  possible,  was  cast  on  a  leaf  and  kept  steadily  in  the  same  place  for  some 
hours.  Thus  the  chlorophyll  could  absorb  energy  only  in  those  regions  along  the 
band  of  light  where  fell  the  waves  of  lengths  that  it  can  stop;  on  the  leaf  these  re- 
gions of  course  corresponded  in  position  to  the  absorption  bands  before  described. 
If,  therefore,  the  leaf  works  with  the  absorbed  energy,  photosynthesis  can  occur  only 
in  these  strips  and  not  elsewhere.  After  the  exposure,  on  testing  the  leaf  for  starch 
(the  accumulation  of  which  is  a  mark  of  active  photosynthesis),  it  was  found  in 
abundance  where  lay  absorption  band  1  (fig.  648),  and  scantily  in  others;  but 
it  was  wholly  lacking  in  other  parts  of  the  spectrum. 

This  is  what  would  be  expected;  but  there  was  once  an  idea  that 
chl<  rophyll  acted  merely  as  a  screen,  shading  the  protoplasm  from  harm- 
ful rays  of  light;  and  that  the  protoplasm  could  work  properly  only 
behind  such  a  screen.  There  is  now  evidence  that  the  protoplasm  is 
unnecessary  in  the  first  stages  of  carbohydrate  synthesis,  those  strictly 
called  photosynthesis.  It  is  probably  light  transformed  to  electricity 
that  reduces  the  H2COs  to  formaldehyde  (see  p.  375),  which  then  con- 
denses into  more  complex  carbohydrates. 

Exposure  to  light.  —  Plainly  the  light  which  has  passed  through  a 
chlomplast  is  unlike  that  which  has  not;  and  the  more  chloroplasts  it 
passes  through,  the  more  complete  is  the  absorption  of  effective  waves. 
The  upper  cells  of  a  leaf,  therefore,  are  in  a  more  favorable  position  with 
respect  to  light  than  the  lower,  especially  in  weak  or  diffuse  light ;  but 


NUTRITION  371 

if  the  stomata  are  only  on  the  under  surface,  as  they  often  arc,  the  lower 
cells  arc  more  favorably  placed  with  respect  to  ('()„;  and  the  more  soas 

the  looser  arrangement  of  these  cells  permits  freer  diffusion.  The  very 
structure  of  the  leaf  is  in  large  measure  a  response  to  these  different 
factors,  and  so  perhaps  the  advantages  and  disadvantages  balance  one 

another.  A, leaf  which  is  directly  shaded  by  another  is  obviously  in  a 
decidedly  disadvantageous  situation;  and  we  observe  various  arrange- 
ments and  positions  that  reduce  shading.  These  result  in  leaf  mosaics 
of  various  kinds  (sec  Part  III,  p.  543).  A  plant  that  grows  in  shade 
is  different  from  the  same  species  grown  in  the  sun;  indeed  shade  plants 
have  peculiarities  which  depend  in  large  part  on  the  difference  in  the 
illumination  (see  Part  III,  p.  531). 

Energy  obtained.  —  An  ordinary  thin  leaf  reflects  and  absorbs  40-70 
per  cent  of  the  sunlight  which  falls  upon  it  ;  but  of  diffuse  light  it  absorbs 
about  95  percent.  The  chlorophyll  itself  seems  to  absorb  something  like 
20-30  per  cent,  but  of  this  only  a  small  part  can  be  used  for  photo- 
synthesis and  so  stored  as  potential  energy  in  the  carbohydrate  made. 
That  amount  is  variously  estimated  from  0.5  to  3  per  cent.  The  balance 
is  free  to  heat  the  leaf,  whose  internal  temperature  in  the  sun  sometimes 
rises  10-150  above  that  of  the  air.  This  surplus  heat,  of  course,  is  partly 
transferred  to  the  air  adjacent,  but  the  greater  part  becomes  latent  in  the 
water,  whose  vaporization  is  accelerated  thereby.  This  is  the  so-called 
"  chlorovaporization  "  (see  p.  330). 

Deficiency  in  light.  —  It  will  be  evident  from  the  foregoing  that  in 
nature  light  is  seldom  lacking  to  drive  the  machinery  rapidly  enough  to 
dispose  of  all  available  C02.  Yet  it  may  be  reduced  to  an  intensity  at 
which  light,  instead  of  the  small  supply  of  C02,  limits  the  output.  For 
example,  some  plants  are  so  situated  that  they  get  only  2  per  cent  of  the 
total  sunlight  in  the  vicinity.  From  the  point  at  which  the  effective 
energy  of  the  light  absorbed  is  just  equal  to  disposing  of  the  available 
CO...,  whether  this  is  greater  than  natural  or  not,  lessening  the  intensity 
of  the  light  results  in  a  proportional  diminution  of  the  amount  of  the 
product. 

Efficiency.  —  It  will  be  further  evident  that  the  plant  is  a  very  in- 
efficient machine-,  considering  the  relation  of  energy  received  to  the 
energy  stored  in  the  produ(  t.  A  -team  engine  which  delivers  as  mechan- 
ical power  less  than  10  per  c  cut  of  the  energy  of  the  fuel  consumed  under 
tin-  boilers  is  lit  for  the  s(  rap  heap,  and  the  best  types  are  delivering  above 
1  5  per  tent.     ( lontrast  this  with  the  O.5-3  l"-'r  l rnl  "'  ''u'  |''anl  '''  oiiomy. 


372  PHYSIOLOGY 

Yet  in  spite  of  this  relative  inefficiency,  the  total  product  is  enormous 
and  invaluable,  because  of  the  limitless  store  of  energy  pouring  upon 
the  earth  constantly  from  the  sun,  beside  which  the  artificially  released 
energy  of  fuel  is  absolutely  a  negligible  quantity. 

The  solar  energy  received  by  the  earth  in  a  second  is  represented  by  250  X  io15 
calories.  The  coal  consumed  in  the  whole  world  in  a  year,  reported  in  1906  as  about 
1000  million  metric  tons,1  represents  8  X  io15  calories.  The  plant  can  afford,  so  to 
speak,  to  be  inefficient. 

Source  of  light.  — The  source  of  light  is  quite  a  matter  of  indifference. 
In  nature,  of  course,  the  primary  source,  the  sun,  is  alone  to  be  con- 
sidered, since  the  light  of  even  the  full  moon  (only  -g^oVo^  ^at  °f  tne 
sun)  is  too  weak  to  effect  photosynthesis  to  a  measurable  extent.  Va- 
rious secondary  sources  may  be  used  in  experiments,  some  electric  lamps 
and  the  incandescent  mantles  (with  gas)  giving  light  of  sufficient  inten- 
sity when  near  the  plants.  Attempts  to  "  force  "  plants,  by  enabling 
them  to  make  food  by  night  with  electric  arc  illumination,  have  been 
successful  with  certain  sorts,  showing  that  there  is  no  need  for  rest  at 
night,  and  that  a  greater  supply  of  food  permits  more  rapid  develop- 
ment; but  there  will  be  no  incentive  for  commercial  application  of  this 
result  until  the  cost  of  electric  energy  is  vastly  less  than  now. 

Temperature.  — A  suitable  temperature  has  usually  been  considered 
merely  a  condition  of  photosynthesis,  and  not  a  source  of  energy  for 
the  process.  This  is  evidence  that  our  knowledge  of  the  energy  rela- 
tions of  this  process  is  vague,  and  that  the  matter  needs  investigation. 
At  present,  however,  it  is  not  possible  to  describe  in  terms  of  energy 
the  effect  of  heat  upon  photosynthesis,  so  we  must  be  content  with  a 
brief  statement  on  temperature  as  a  condition. 

Experiments  show  that  even  at  temperatures  approaching  o°  C. 
some  plants  can  make  carbohydrates;  the  algae  of  arctic  waters  are 
conspicuous  examples.  Yet  for  most  plants  such  a  low  temperature 
practically  stops  photosynthesis;  while  even  at  several  degrees  higher 
it  may  be  the  limiting  factor,  less  food  being  made  than  the  COL,  and 
light  would  permit.  Likewise  in  direct  sunlight  the  temperature  may 
rise  so  high  in  the  interior  of  a  leaf  as  to  retard  photosynthesis2;  and 
in  tropical  deserts,  where  the  heat  of  the  air  itself  may  run  to  450  C, 
it  is  probable  that  photosynthesis  is  reduced  thereby. 

1  The  metric  ton  about  equals  the  English  "long  "  ton,  2200  lbs. 

2  But  these  heating  effects  of  direct  sun  are  compensated  in  a  measure  by  evaporation. 


NUTRITION  373 


(4)    The  Products  and  the  Process 

The  products.  — The  first  product  of  photosynthesis  is  not  known  with 
entire  certainty,  and  the  process,  therefore,  cannot  be  described  ac- 
curately. The  product  of  later  synthesis  whi<  h  is  most  general  and  has 
been  longest  known  is  starch.  The  fart  that  it  is  so  generally  present 
and  that  it  is  so  universally  used  as  evident  e  of  photosynthesis  be<  ause 
it  can  be  so  easily  detected,  tend  to  confirm  the  common  impression  that 
starch  is  the  producl  of  photosynthesis.  But  there  are  many  plants  in 
which  starch  is  either  not  formed  at  all,  or  appears  only  under  excep- 
tional conditions,  and  in  no  plants  is  it  the  exclusive  product.  Thus,  in 
most  fungi  no  starch  is  formed  when  they  are  fed  on  carbohydrates;  in 
the  kelps  fucosarj  takes  its  place,  and  in  many  monocotyledons,  oil; 
while  even  in  the  plants  which  produce  starch  abundantly,  much  of  the 
earlier  product  is  diverted  into  amides  and  possibly  other  nitrogenous 
compounds. 

In  any  event  starch  is  a  secondary  product,  and  represents  the  surplus 
in  the  manufacture  of  primary  carbohydrates  over  immediate  use,  re- 
moval, transformation  into  amides,  etc.  That  starch  does  not  appear 
under  certain  conditions,  in  a  leaf  in  which  it  is  usually  formed,  is  no 
evidence,  therefore,  that  no  photosynthesis  has  occurred,  but  only  that 
it  has  not  gone  on  at  a  rate  rapid  enough  to  yield  enough  excess  to  appear 
as  starch. 

Amount  of  product.  ■ — A  method  of  estimating  the  amount  of  photo- 
synthesis under  various  conditions  is  based  upon  the  relative  weight 
of  equal,  hut  necessarily  small,  areas  of  leaves,  taken  at  the  be- 
ginning and  end  of  the  experimental  time,  allowances  being  made 
for  migration1  and  use  by  data  from  other  experiments.  The  results 
at  best  cannot  be  exact,  and  the  introduction  and  multiplication  of 
small  initial  errors  make  the  calculations  based  on  these  data  quite 
unreliable. - 

When  accurate  data  for  photosynthesis  are  needed,  the  only  reliable 
method  is  to  determine  the  amount  of  ('< ).,  used.  This  requires  rather 
complicated  apparatus,  skillful  manipulation,  and  accurate  gas  analysis. 
This  method  is  obviously  independent  of  the  products  and  their  use  or 
migration. 

1  <>r  this  may  1m-  rendered  impossible  by  severing  the  leaf  from  the  plant 

2  Tlic  results  obtained  by  this  method  an   twotothrei  times  .1-  large  as  those  in  the 

table  on  the  following  p.iyc. 


374 


PHYSIOLOGY 


The  best  estimates  as  to  the  amount  of  photosynthesis  carried  on  by 
thin-leaved  plants  are  given  in  the  following  table: 


Carbohydrate  made  in  i  hr.  by  i  sq.m.  of  leaf  surface 


Name  of  plant 

Condition 

OF  LEAF 

Light 

Temp. 
°C. 

co2 

USED, 
CC. 

co2 

USED, 
MG. 

Carb. 

MADE, 

i.  Helianthus  animus  .     . 

attached 

diffuse 

21. 1 

312.6 

6l2 

392 

2.  Helianthus  annuus  .     . 

detached 

diffuse 

19.0 

439-9 

S62 

551 

3.  Helianthus  annuus  .     . 

detached 

J  strong  to 
[  diffuse 

26.S 

385-3 

755 

483 

4.  Helianthus  annuus  .     . 

attached 

bright  sun 

47-1 

21.9 

43 

27 

5.  Tropaeolum  ma  jus  .     . 

detached 

diffuse 

21.7 

158.3 

3io 

I98 

6.  Tropaeolum  majus  .     . 

detached 

diffuse 

25-9 

243-7 

487 

305 

7.  Catalpa  bignonioides    . 

detached 

interm.  sun 

20.0 

373-2 

737 

468 

8.  Petasites  albus     .     .     . 

detached 

in  term 

.  sun 

17.0 

208.4 

408 

26l 

9.  Polygonum  Weyrichii   . 

detached 

21.0 

473-2 

927 

593 

10.  Prunus  Laurocerasus    . 

detached 

10.0 

2S1 

11.  Prunus  Laurocerasus    . 

detached 

37-5 

810 

12.  Helianthus  annuus  .     . 

detached 

19.0 

569 

13.  Helianthus  annuus  .     . 

detached 

29.0 

650 

14.  Helianthus  annuus  .     . 

detached 

3S-o 

73° 

Nos.  1-9,  after  Brown  and  Escombe,  in  part  recalculated ;  nos.  10-14,  after 
Blackman  and  Matthaei,  especially  intended  to  show  the  effects  of  temperature 
on  photosynthesis.     An  effect  of  excessive  temperature  is  to  be  seen  also  in  no.  4. 

Using  such  results  as  the  basis  of  calculation,  it  would  be  easy  to  show 
how  enormous  a  weight  of  food  is  made  in  a  growing  season  by  the  foli- 
age of  meadows  and  forests.  But  unknown  allowances  must  be  made  for 
leaves  unfavorably  situated  or  lacking  in  vigor,  and  such  estimates  are 
of  little  value  except  for  their  impressiveness.  The  value  and  volume 
of  the  annual  crops  of  cultivated  plants  is  even  more  impressive;  and 
to  this  must  be  added  in  imagination  the  unknown  but  huge  volume  of 
wild  vegetation,  all  dependent  upon  photosynthesis  for  at  least  85  per  cent 
of  its  dry  substance. 

The  following  are  the  approximate  values  of  some  of  the  more  important  crops 
of  1909  in  the  United  States:  corn,  $1,720,000,000;  wheat,  oats,  rye,  and  barley, 
$1,280,000,000;  cotton,  $850,000,000;  hay,  $665,000,000;  potatoes,  $212,000,000. 
Together  the  weight  of  these  marketable  products  is  something  like  175,000,000 
metric  tons ;  and  of  course  this  is  but  a  small  fraction  of  the  vegetation  that  pro- 


NUTRITION  375 

duced  them.  In  addition  to  the  staple  crops  just  named,  whose  aggregate  value 
in  1909  was  about  $^,000,000,000,  other  farm  crops  add  nearly  as  much  more, 
being  estimated  at  Sj,  700,000,000.  Su<  li  are  the  values  that  plants  annually  pro- 
duce in  this  country,  chiefly  from  the  air  and  water,  by  photosynthesis. 

Process. — The  process  of  photosynthesis  is  not  certainly  known; 
but  all  the  evidence  points  Strongly  in  one  direction;  so  that  the  hypoth- 
esis of  von  Baeyer  may  be  considered  as  highly  probable.  It  appears 
that  the  carbonic  acid  (C02  +  H^O  ^OHCOOH)  is  by  some  means 
reduced,  perhaps  first  to  formic  acid  (HCOOH),  and  later  to  the  sim- 
plest carbohydrate,  formaldehyde  (H-COH).  In  the  course  of  this 
reduction  a  molecule  of  oxygen,  O,,  is  set  free  and  appears  as  a  by- 
product. 

The  reduction  of  KfoCOs  to  formaldehyde  lias  lately  been  accomplished  artifi- 
cially, though  much  less  efficiently  than  in  plants.  A  thin  layer  of  chlorophyll  on 
gelatin  or  floating  on  the  surface  of  water  (to  which  has  been  added  an  enzyme  that 
will  break  up  hydrogen  peroxid,  EI3O2,  into  water  and  oxygen),  when  supplied  with 
CO2  in  light  permits  the  accumulation  of  formaldehyde  and  oxygen  to  a  measurable 
extent  in  the  apparatus,  The  formaldehyde  molecule  so  quickly  combines  with 
others  of  its  kind  thai  it  has  been  difficult  to  prove  its  formation  in  leaves.  Free, 
it  is  a  powerful  poison,  even  in  dilute  solution  (1  :  20,000);  but  its  prompt  conden- 
sation into  some  hexose  sugar  prevents  accumulation  to  a  harmful  extent.     The 

/H 
details  are  probably  as  follows:  six  molecules  of  formaldehyde,  II— C^.       ,  unite 

into  a  chain.  This  union  engages  two  of  the  four  bonds  of  each  C  atom,  except  at 
the  ends,  where  only  one  is  concerned.  This  consequently  either  releases  one  of 
the  twoO  bonds  or  leaves  one  H  atom  free,  or  does  both.  The  free  II  immediately 
joins  its  neighboring  half-free  O,  and  together  they  form  <  >II,  bound  t<>  C  by  only 
one  bond.  At  one  end  no  II  is  freed  ;  but  the  half-freed  (  )  takes  up  H  and  the  group 
becomes  CH2OH,  characteristic  of  an  alcohol.     At  the  other  end,  the  loss  of  one 

II    leaves  the  aldehyde  group  C^       as  in  formaldehyde.     In  glucose  a  further 


% 


O 


transposition  occurs  in  group  4,  II  and  OH  exchanging  places. 

H    H    H  OH   H 
I       I       I       I       I         /H 
H— C— C— C-C- C     C<       =  ./-glucose  (p.  159), 

OHOHOH  II   OH 

Glucose  and  starch. — Glucose  probably  represents  the  first  stable 
carbohydrate  formed  in  most  plants;  yet  there  i^  some  variation  in  this 
respect  in  different  plants,  and  there  is  evidence  thai  in  some  cases  cane 
sugar,  saccharose,  is  the  thief  product.     It  is  quite  possible,  moreover, 


376  PHYSIOLOGY 

to  divert  some  of  the  product  into  amides  by  a  simple  substitution  of  the 
amide  radical,  NH2,  for  some  H  or  OH  radical.  Thus,  if  the  fifth  group 
in  the  glucose  chain  became  HC(NH2),  the  product  would  be  glucosamin, 
a  substance  of  quite  different  properties  (see  p.  360).  Like  diversion  by 
substitution  might  readily  occur  if  only  two  or  three  formaldehyde  mole- 
cules had  come  together.  Such  processes  seem  to  be  the  initial  steps  in 
protein  synthesis  (p.  380). 

The  common  main  product,  glucose,  usually  accumulates  in  the  cells 
because  it  is  formed  faster  than  it  can  move  away.  Finally  starch  or 
some  other  stable  product  appears.  The  intervening  steps  are  hypo- 
thetical. It  seems  that  at  a  certain  concentration  glucose  molecules  show 
a  tendency  to  combine  with  each  other  to  form  a  compound  sugar,  maltose 
(CioHwOn),  which  promptly  compounds  itself  in  like  manner  into  a 
dextrin  and  finally  into  starch.  The  combinations  occur  rapidly,  and 
the  intermediate  products  are  hence  obscure.  Perhaps  the  process  takes 
place  under  the  influence  of  third  bodies, 
called  enzymes;  maltase  and  diastase  in 
the  cases  here  cited  being  the  possible 
agents  (but  see  enzymes,  p.  399).  The 
Fig.  649.-TW0  chioToplasts  starch  accumulates  in  minute  granules 
of  Rhipsalis,  with  grains  of  starch  within  the  chloroplasts  (fig.  649),  so  their 
(5)  and  minute  oil  droplets.  —  stroma  may  be  the  direct  agent  in  organ- 
izing the  starch,  or  at  least  may  be  the 
seat  for  the  formation  of  the  enzymes  which  bring  this  about.  These 
grains  have  a  definite  structure  and  a  rather  uncertain  composition  (see 
starch,  p.  358),  for  both  of  which  the  chloroplast  itself  may  be  responsible 
(see  leucoplasts,  p.  389). 

Removal  of  products.  —  If  a  leaf  is  isolated,  the  accumulation  of  the 
synthetic  products  may  reach  a  point  where  it  interferes  with  further 
photosynthesis;  but  in  nature  this  does  not  occur.  Use  on  the  spot,  or 
diffusion  of  such  products  as  remain  simple  and  soluble,  or  the  digestion 
of  the  more  complex  and  the  insoluble  ones  by  enzymes  (p.  399)  and 
subsequent  diffusion,  is  constantly  removing  the  new  materials  from  the 
leaves  and  stems  to  other  places  where  they  may  accumulate  or  be  used 
(see  translocation,  p.  393).  In  darkness  or  weak  light,  the  transporta- 
tion facilities,  temporarily  overtaxed  in  full  light,  overtake  the  manu- 
facturing;  the  laboratories  are  cleared,  consumers  are  supplied,  and  the 
warehouses  and  distributing  centers  are  filled  with  the  surplus  awaiting 
future  use. 


NUTRITION 


377 


The  by-product.  —The  by-]  >rod- 
uct, oxygen,  is  used  to  some  extent 

in  respiration  (p.  406);  the  excess 
diffuses  to  the  surface,  whence  it 
escapes  into  the  aerating  system 
and  theme  into  the  air.  The  final 
step  in  its  exit  i  an  be  observed  in 
water  plants  readily,  because  the 
constant  accumulation  in  the  air 
chambers  leads  to  its  escape  as 
bubbles  when  the  passages  are 
opened  by  a  cut  or  break  (fig.  650). 
If  the  canals  are  intact,  02  may 
become  abundant  enough  in  bright 
light  to  form  bubbles  on  the  sur- 
face, which  rise  as  they  become 
larger.  The  rising  gases  can  be 
conducted  by  an  inverted  funnel 
into  a  test  tube  and  analyzed; 
they  are  about  85  per  cent  oxygen, 
the  remainder  being  other  gases 
produced  in  other  processes.  So 
uniform  is  the  evolution  of  02  by 
water  plants  that  with  precautions 
the  number  of  bubbles  given  off 
in  unit  time  can  be  used  to  exhibit 
the  general  effect  of  the  three  ex- 
ternal fat  tors,  intensity  of  light, 
temperature,  and  supply  of  C02, 
on  photosynthesis.  It  is  not  sat- 
isfactory for  quantitative  deter- 
minations. 


Fir..    650.  —  Upper    part    of    a    plant   of 

ton  attached   to  a  glass  rod   and 

submersed,  showing  escape  of  gas  bubbles 

(mostly   oxygen)   from   cut   end   of    stun    in 
sunlight. 


3.   THE    SYNTHESIS    OF    PROTEINS 

Proteins  the  end-product.  —The  formation  of  carbohydrates  is  by  no 
mean-  the  only  process  of  food  making.  Indeed  it  may  be  looked  upor 
a>  merely  the  firsl  stage  in  the  construction  of  proteins,  of  which  carbo- 
hydrates are  important  components.     As  the  living  protoplasm  appears 


378  PHYSIOLOGY 

to  be  composed  chiefly  of  proteins  (probably  more  complex  and  labile 
than  in  the  non-living  state),  it  is  evident  that  protein  foods  are  of  the 
highest  importance  —  indeed  indispensable  —  for  nutrition,  since  it  is 
the  protoplasm  which  grows,  wastes,  and  needs  repair.  Proteins  are, 
as  it  were,  the  highest  type  of  foods;  they  represent  the  final  stage  of 
food  making. 

Inasmuch  as  the  carbohydrates  contain  only  carbon,  hydrogen,  and 
oxygen,  while  proteins  contain  in  addition  nitrogen  and  sulfur  and  in 
many  cases  phosphorus  also,  it  is  plain  that  they  cannot  be  formed  from 
carbohydrates  alone.  A  strict  carbohydrate  diet  is  as  unsuitable  for 
plants  as  it  is  for  animals.  Some  materials  must  be  supplied  from  which 
nitrcgen,  sulfur,  and  phosphorus  can  be  obtained. 

Source  of  nitrogen.  —  As  the  air  contains  78  per  cent  of  nitrogen,  the 
atmosphere  would  appear  to  be  a  natural  source  of  this  element.  But 
though  the  nitrogen  is  everywhere  dissolved  in  the  water  of  the  plant, 
and  can  enter  and  leave  it  freely,  no  plants  are  known  to  be  able  to  use 
it  in  this  uncombined  form,  except  certain  bacteria,  some  of  which  live 
in  the  soil  and  in  some  waters.  Certain  soil  species  enter  the  roots  of 
various  plants,  especially  the  Leguminosae,  causing  them  to  form  tuber- 
cles. (See  below,  p.  379.)  Almost  all  plants,  therefore,  must  get  com- 
bined nitrogen.  This  is  found  in  soils  as  nitrates  of  various  bases,  e.g. 
calcium,  magnesium,  potassium,  and  sodium;  and  when  a  soil  is  deficient 
in  nitrogen,  such  compounds  are  important  constituents  of  the  fertilizers, 
natural  and  artificial,  which  are  added  to  it.  The  nitrates  in  the  soil 
result  mainly  from  the  decay  of  organic  matter  in  it.  The  later  steps 
in  the  process  are  controlled  by  certain  bacteria  in  the  soil  which  bring 
about  the  oxidation  of  ammonia  to  nitrites,  whereupon  others  oxidize 
the  nitrites  to  nitrates.  The  very  fertility  of  arable  soils,  therefore, 
depends  on  the  microscopic  organisms  living  in  them,  which  prepare  the 
way  for  the  larger  plants. 

Loss  of  N.  — The  soil  of  cultivated  areas  is  constantly  losing  its  com- 
bined nitrogen  by  solution  and  drainage,  and  this  loss  is  only  partially 
made  good  by  the  ammonia  and  nitrous  and  nitric  acids  washed  into  it 
from  the  air  by  rains.  Under  natural  conditions  the  dying  vegetation 
ultimately  returns  its  constituents  to  soil  and  air;  but  crops  are  carried 
off,  their  nitrogen  with  them.  Gardens  and  fields,  therefore,  require 
replacement  of  this  nitrogen  sooner  or  later.  When  they  lie  fallow, 
certain  bacteria  of  the  soil,  associated  with  algae  and  perhaps  with 
other  plants,  slowly  increase  the  nitrogen  content  of  the  soil  by  fixing 


NUTRITION  379 

the  free  N2  from  the  air  in  their  bodies,  which,  dying,  restore  it  to 
the  soil. 

Leguminosae.  — The  case  of  the  Leguminosae  and  a  few  other  plants 
i-  peculiar.  Certain  soil  bacteria  cuter  the  young  root  hairs,  grow 
and  multiply,  and  work  gradually  into  the  cortex,  where,  as  they  in<  rease, 
they  stimulate  the  rootlet  to  multiply  and  enlarge  the  cortical  cells,  so 

that  a  local  swelling  or  tubercle  is  formed.  The  largest  of  these  a  an  civ 
exceed-  the  size  of  a  hazelnut,  and  most  are  smaller  than  a  pea  or  even 
a  grain  of  wheat.  The  relations  are  probably  as  follows:  The  bat  teria 
depend  on  their  host  for  carbohydrate  food,  but  can  use  the  free  nitrogen 
(presumably  that  nearest  them  in  solution,  which  is  replaced  from  the 
air)  in  their  protein  making.  Being  favorably  situated,  many  of  the  bac- 
teria become  excessively  enlarged,  and  often  branch  into  X  and  Y  forms. 

The  host  sooner  or  later  gets  the  better  of  the  parasite  and  consumes 
these  fat  bacteria  ("  bacteroids  "),  their  proteins  proving  valuable  foods. 
By  reason  of  this  peculiar  relation,  leguminous  crops  can  be  grown  in 
soils  which  contain  no  combined  nitrogen  whatever,  provided  the  proper 
bacteria  be  present.1  If  the  crop  be  then  plowed  under  (a  process 
called  green  manuring),  the  soil  is  enriched  in  nitrogen  at  the  expense 
of  the  air.2     (See  further  Part  III  on  root  tubercles.) 

Source  of  S  and  P.  — The  sulfur  and  phosphorus  needed  are  obtained 
by  the  green  plants  from  sulfates  and  phosphates  which  dissolve  in  the 
soil  water.  Few  soils  lack  these,  though  for  cropping  the  phosphates 
may  be  insufficient  or  may  be  so  reduced  as  to  interfere  with  full  devel 
opment.  "  Land  plaster  "  (gypsum,  or  calcium  sulfate)  is  sometimes 
applied  to  fields;  but  it  probably  has  more  beneficial  effects  on  other 
qualities  than  on  the  composition  of  the  soil.  Phosphates  are  an  impor- 
tant part  of  artificial  manures.2  In  the  case  of  both  nitrogen  and  phos- 
phorus it  is  highly  important,  if  immediate  effects  are  desired,  that  the 
compounds  be  such  as  are  "  available,"  and  compounds  can  be  available 
only  when  they  are  soluble  or  readily  become  so. 

Raw  materials.  — The  nitrates,  sulfates,  and  phosphates  enter  the 
larger  plants  through  the  roots.  These  are  the  mineral  salts  which  are 
most  necessary  for  the  wclbbcing  of  the  plant,  because  they  are  needed  for 

1  If  qoI  the  soil  may  be  info  ted  by  scattering  on  it  soil  in  which  such  a  crop  has  been 
previously  thrown.     Commercial  attempts  t<>  supply  pure  cultures  of  appropriate  bacteria 

for  infecting  the  -oil  through  the  seed  sown  have  not  been  very  successful. 

■The whole  Subject  of  the  relation  of  manures  and  fertilizers  to  the  soil  and  crop  is  m 
a  very  unsatUfat  tory  state  and  needs  further  investigation  before  the  practice  and  results 
ran  he  explained. 


380  PHYSIOLOGY 

protein  synthesis.  LikeCOo  and  H20,  they  have  been  called  "  foods  "; 
but  it  is  far  better  to  look  upon  them  as  raw  materials  out  of  which,  with 
others,  food  can  be  made. 

Given  carbohydrates  (finished  and  partly  torn  up  again,  or  "  in  the 
making  ")  plus  nitrates,  sulfates,  and  phosphates,  most  plants  can  make 
proteins.  There  is  no  set  of  plants  to  whiclv  protein  synthesis  is  re- 
stricted, as  is  photosynthesis  to  the  green  plants.  Yet  there  are  plants 
(certain  bacteria  for  example)  which  require  their  nitrogen  supplied  in 
other  forms  than  nitrate,  and  some  even  which  can  use  nothing  less 
complex  than  proteins.  Here  we  may  properly  speak  of  assimilation 
rather  than  of  synthesis. 

No  special  organs.  —  In  the  larger  plants  protein  synthesis  is  not  re- 
stricted to  a  particular  organ.  Neither  chlorophyll  nor  light  is  essential 
to  it,  for  it  is  carried  on  freely  by  fungi  which  have  no  chlorophyll,  and 
it  is  doubtful,  in  spite  of  much  experimenting,  whether  light  has  any  in- 
fluence upon  its  rate.  Since  carbohydrates  are  usually  the  basis  of  pro- 
tein synthesis,  the  leaves,  in  green  plants,  are  the  chief  seat  of  this  pro- 
cess; for  in  the  leaves  carbohydrates  are  being  made,  and  to  them  stream 
the  dilute  watery  solutions  of  salts,  brought  via  the  xylem  bundles  by 
evaporation. 

Process.  —  So  long  as  the  constitution  of  proteins  remains  unknown 
it  will  be  impossible  to  describe  the  process  by  which  they  are  made. 
Inasmuch  as  all  proteins  on  decomposition  yield  amides  (amino-acids), 
and  the  simpler  ones  are  certainly  formed  from  them  by  condensation, 
it  is  supposed  that  carbohydrates  are  converted  into  amides  first,  by  the 
introduction  of  NH2-groups  here  and  there,  and  that  these  amides  link 
themselves  together,  some  becoming  modified  by  the  incorporation  of 
sulfur  and  phosphorus  molecules,  and  so  form  proteins  of  various  kinds. 
But  the  details  are  all  uncertain  and  only  the  vaguest  statements  can  be 
made. 


4.    OTHER   WAYS   OF    GETTING   FOOD 

Dependent  plants.  — The  green  plants  are  sometimes  distinguished 
from  others  by  the  term  autotrophic,  meaning  that  they  nourish  them- 
selves by  their  ability  to  make  in  their  own  bodies  the  most  important 
foods,  the  carbohydrates.  All  others  are  heterotrophic  plants,  signifying 
that  they  secure  food  in  a  different  way.  (But  see  p.  362.)  The  more 
important  ways  are  now  to  be  described. 


NUTRITION 


3«i 


Among  the  many  thousand  species  of  heterotrophic  plants,  the  bac- 
teria and  fungi  hold  the  dominant  place.  A  few  seed  plants  lack 
chlorophyll  entirely,  such  as  the  Indian  pipe  (Motwtropa),  beech  drops 
(Epifagusvirginiana),  dodder  (Cuscuta),  etc.;  and  some  have  only  par- 
tially lost  it,  or  with  a  good  supply  nevertheless  have  the  nutritive  habits 
of  the  non-green  plants. 

The  families  in  which  such  dependent  species  are  prominent  are  theLoranthaceae, 
Rafflesiaceae,  Scrophulariaceae,  <  >rol>am  hat  ear,  and  Halanophora  eae. 

If  a  plant  cannot  make  carbohydrates,  it  must  of  necessity  get  food 
directly  or  indirectly  from  some  plant  that  can.  The  direct  way  of 
doing  this  is  to  live  on  or  in  a  live  green  plant.  The  indirect  way  differs 
only  in  that  the  food  secured  is  more  remote  from  the  original  food 
maker.  Thus,  a  plant  may  live  upon  or  in  some  animal  or  some  non- 
green  plant,  or  upon  the  dead  bodies  of  these,  more  or  less  decayed  and 
disintegrated.  Indeed,  decay  and  disintegration  are  only  the  obvious 
evidence  that  plants  (chiefly  the  minute  bacteria  and  fungi)  are  living 
upon  such  a  dead  body.  And  not  infrequently 
death  itself  is  simply  the  result  of  the  vigorous 
development  of  such  creatures  on  or  in  the 
body  of  a  once  healthy  organism. 

Parasitism.  —  An  association  between  two 
live  organisms  is  known  as  symbiosis.  When 
one  obtains  its  food  from  the  other,  the  rela- 
tion is  called  parasitism,  and  the  two  are  known 
respectively  as  parasite  and  host.  As  a  rule 
the  food  maker  is  called  the  host,  and  the  other 
the  parasite;  if  neither  or  both  be  food  makers, 
the  larger  is  distinguished  as  the  host.  Thus, 
fungi  are  parasitic  on  leaves  or  twigs  or  in 
the  wood  of  trees,  or  on  animals;  "  beech- 
drops  "  (Epifagus  virginiana,  a  small  flower- 
ing plant)  is  parasitic  on  the  roots  of  the  beech 
tree;  mistletoe  is  parasitic  on  elms,  etc.  This 
relation  requires  the  closest  contact  between 
the  cells  of  parasite  and  host,  and  the  parasite 
even  penetrates  the  1  ells  of  the  host  in  many 
cases.  The  smaller  parasites,  such  as  fungi, 
may  grow  bodily  through  cells,  doubtless  dis- 


FlG.  651. — An  epidermal 
cell  of  a  grass  |  Poo)  penetrated 
l>y  a  branched  naustorium  ^1) 
of  a  fundus  (Erysiphe  grami- 
nis);  the  mycelial  hypha  to 
which  the  slender  penetrating 
tube  (a)  is  attached  is  not 
shown.  —  After  Smith. 


382 


PHYSIOLOGY 


Fig.  652.  —  Section  of  stem  penetrated  by  haustorium 
(h)  of  dodder  (Cuscuta). — From  Part  III.  (For  ex- 
planation of  letters,  see  fig.  1082.) 


solving  the  wall  by  some  enzyme  (see  digestion,  p.  399),  or  it  may  send 
into  them  short  branches,  called  haustoria  (tig.  651;   see  also  figs.  1079, 

1080,  Part  III),  through 
which  the  food  enters 
the  parasite.  A  vascu- 
lar parasite,  the  dod- 
der, which  twines  exten- 
sively over  coarse  herbs, 
sends  into  its  host  short 
branches,  likewise  called 
haustoria  (fig.  652), 
whose  vascular  strands 
come  into  the  most  inti- 
mate contact  with  those 
of  the  host.  (See  Part 
III  on  parasitism.) 

Partial  parasites.  — 
When  such  complete 
contact  has  been  estab- 
lished, it  is  difficult  to  determine  what  or  how  much  material  migrates 
from  host  to  parasite.  Colorless  parasites,  of  course,  must  get  all 
their  food  from  the  host.  Certain  green  parasites 
undoubtedly  could  live  by  getting  merely  water  and 
its  dissolved  salts,  for  they  can  make  food  for  them- 
selves. Hence  they  are  known  as  partial  parasites. 
But  that  they  completely  restrict  themselves  to  such 
food  materials  and  do  not  admit  any  real  food  is 
quite  improbable,  in  view  of  the  intimate  union 
between  the  two. 

Mutualism.  —  The  support  of  the  parasite  by  the 
host  may  result  in  no  considerable  injury  or  even 
weakening.  Indeed,  many  cases  have  been  described 
in  which  the  association  suggested  a  partnership, 
whence  the  term  mutualism.  From  another  point 
of  view  the  relation  resembles  that  of  master  and  European  beech  (Fa- 
slave,  whence  the  term  helotism  (see  Part  III).  £"s  ^^a);h,  hy- 
'  v  '       phae.  —  After  Frank. 

The  lichens  (p.   78)  furnish  the  classical  example. 

Yet  even  here  the  algae  are  somewhat  restricted  in  development  by 
the  constant  drain  upon  them,  though  perhaps  they  can  work  at  food 


Fig.  653.  —  Ecto- 
trophic  mycorhiza   of 


NUTRITIi  >N 


3*3 


making  longer  because  the  encompassing  fungus  by  its  spongy  tex- 
ture retains  rainwater  longer  than  would  the  algae  alone.  Mycorhiza 
i>  another  instance  of  so-called  mutualism,  in  which  fungi  associate 
themselves  with  the  roots  of  certain  plants, 

especially  the  oaks  (Cupuliferae),  the  heaths 
(Kricaceae),  and  the  orchids  (Orchidaceae). 
Sometimes  they  jacket  the  rootlets  with  a 
weft  of  filaments  (cctotrophic  mycorhiza,  fig. 
653),  and  sometimes  they  penetrate  the  corti- 
cal cells,  forming  a  tangle  about  the  nucleus 
(endotrophic  mycorhiza,  fig.  654).  The  fungi 
are  supposed  to  aid  the  root  in  acquiring 
water  and  food  materials  (especially  nitrogen 
compounds,  which  they  themselves  may  form 
from  the  free  nitrogen  of  the  air)  from  the 
soil.  Certainly  they  derive  some  food  from  F.G.  6j4  _  Endotrophic 
the  root,  and  injury  to  the  root  is  suggested  mycorhiza  of  Neotiia:  a,  host 

,       .  11       r  1      1       e  .1  cell  with  active  fungus  hyphae; 

by  its  stubby  form  and  the  frequent  absence  b>  ccll  with  dcgcnerating  hy_ 
of  root  hairs.      In  fact,  the  more  the  cases  of   phae.— After  Magnus.    (See 
SO  >alled  mutualism  are  studied,  the  more  it   a  °   gs" 
becomes  evident  that  they  are  only  cases  of  modified  parasitism,  with 
minor  injury  to  the  host.    (See  Part  III  on  reciprocal  parasitism.) 

Injury  by  parasites.  —  On  the  other  hand,  the  drain  on  the  food  re- 
sources of  the  hist  may  be  severe,  so  weakening  it  that  it  succumbs  to 
adverse  conditions  which  otherwise  could  be  overcome.  Quite  apart 
from  this  weakening  for  lack  of  food,  the  parasite  may  act  as  a  stimulus 
to  local  growth,  or  it  may  produce  injurious  substances  which  cause  local 
or  even  general  death.  The  location  of  a  parasite  is  often  marked  by 
deformities;  leaves  are  crinkled  or  thickened,  as  in  peach  curl;  circum- 
si  ril>e<|  swellings  of  peculiar  and  fantastic  or  beautiful  forms  (galls)  grow 
on  leaves  or  stems  (tig.  655);  even  large  tumors  are  formed,  as  in  the 
Maik  knot  of  (  herry  and  plum  trees.  Local  death  is  another  common 
mark  of  the  presence  of  a  parasite.  The  fire  blight  of  apple  and  pear 
tree-,  .hie  to  parasitic  bacteria,  gets  its  name  because  young  shoots 
are  killed  for  a  distance  of  20  to  50  cm.,  and  the  withered  brown  leaves 
make  the  tree  look  as  though  it  had  been  scorched  by  a  tire.  General 
death  in  large  plants  is  seldom  produced  by  a  parasite  unless  it  inter- 
feres with  the  water  supply  or  invades  the  entire  organism  In  wilt 
disease  the  parasite  blocks  the  tracheae,  interfering  with  the  supply  of 


384 


PHYSIOLOGY 


water  to  the  leaves,  and  death  follows  with  surprising  suddenness.  In 
other  cases,  since  in  plants  there  are  no  means  for  quick  distribution  of 
poisons  locally  produced,  nor  any  regulatory  centers  whose  injury  up- 
sets the  whole  system,  death  is  likely  to  be  merely  local.  In  animals, 
on  the  contrary,  a  parasitic  plant,  restricted  to  a  limited  region,  may 
produce  poisons  which  are  quickly  spread  through  the  body  by  the 
blood,  attack  the  central  nervous  system  or  important  viscera,  and 
soon  cause  death.     Thus,  in  diphtheria,  the  bacteria  flourish  chiefly 


Fig.  655.  —  Galls:    a,  on  leaf  of  rose;    b,  on  stem  of  grape.  —  From  Part  III. 

in  the  throat,  where  they  may  produce  no  serious  lesion,  but  the 
toxins  produced  reach  the  heart  and  kidneys  and  sometimes  fatally 
injure  them. 

Saprophytes.  —  The  association  of  a  plant  with  a  dead  organism  or 
organic  debris  is  called  saprophytism,  and  the  live  member  is  a  sapro- 
phyte. Since  a  parasite  may  kill  its  host  and  then  continue  to  live  upon 
the  body,  the  distinction  between  parasites  and  saprophytes  is  not  always 
clear.  Thus  there  are  obligate  parasites  and  obligate  saprophytes; 
plants,  namely,  that  are  obliged  to  live  in  one  relation  or  the  other.  Cor- 
respondingly there  are  facultative  parasites  and  facultative  saprophytes, 
which  may  pass  part  of  their  lives  in  one  way  and  part  in  the  other  or 
wholly  in  either.  Often  the  full  cycle  can  be  completed  only  if  the  given 
plant  can  establish  the  preferred  relation. 


NUTRITION  385 

Saprophytes  are  very  numerous  and  varied.    They  may  be  superfii  ial, 
or  may  penetrate  the  substratum  thoroughly,  showing  finally  at  the  sur- 
face only  the  reproductive  bodies.     The  very  fact  that  they  are  getting 
food    from    the 
dead  organism 
indicates    that 
they    are    con- 
suming it.    In- 
asmuch as  they 

often  must  digest  the  food  before  it  can  enter  their 
bodies,  they  disintegrate  the  body  on  which  they 
feed.  In  the  course  of  this  digestion  and  disin- 
tegration, many  and  varied  chemical  reactions 
occur,  some  incited  by  the  saprophyte,  some  in- 
cidental to  the  changes  it  produces.  These  are 
summed  up  for  fluid  media  under  the  term  fer- 
mentation, and  for  solids  under  the  terms  decay  or 
putrefaction.  Certainly  in  fermentation  (p.  409), 
and  probably  also  in  putrefaction  and  decay,  some 
of   the  most    striking   reactions  are  not  connected  \]k     • 

with  food  getting,  though  apparently  they  are  en- 


tire! v  similar  thereto. 


Organic    debris.  —  It   is  not  necessary  that  the        If ' ','U'u1 
dead   body  retain   any   semblance  of  its  original        iljj   rMjil 
form.     It  may  even  be  so  far  destroyed  as  to  be         m^  if  Vh'j^| 
mere  particles  of  a  soil;   yet  the  saprophyte  relies         wL* *"    U 
on  these  for  its  food.     Thus,  the  common  mush-  W  \     $ 

room  of  comment'  (Agaricus  campestris)  is  grown  \\|    v 

upon  a  compost  of  soil  and  horse  dung,  the  par-  \l  \  v<? 

daily  digested  remnants  of  grain  and  hay  furnish- 
ing the  food  for  the  mycelium.  Indeed,  every  soil 
containing  organic  matter  supports  a  varied  if  piQ  6s6>_Lea|  of  jy* 
minute  flora,  whose  operations  are  often  indispen-  penthes Mastersiatia.— Froma 
sable  to  the  welfare  of  larger  plants.  photograph  by  G.W.Ouver. 

Succession.  —  Nothing  is  more  striking  than  the  succession  of  sap- 
rophytes that  live  upon  a  dead  organism  and  finally  dispose  of  all  its 
organic  matter,  each  appropriating  a  suitable  part  and  reducing  that 
to  the  most  simple  and  stable  compounds,  until  finally  it  "  returns  to  the 
dust  whence  it  came."    This  emphasizes,  too,  the  striking  differences 

C.  B.  &  C.    HOT  ANY  —  25 


386 


PHYSIOLOGY 


between  saprophytes  in  their  use  of  offered  foods  —  differences  which 
at  present  are  quite  inexplicable.  A  classification  of  saprophytes  accord- 
ing to  the  sort  of  food  on  which  they  thrive  best  has  been  made;  but  this 
expresses  only  in  a  summary  way  our  very  imperfect  knowledge  of  their 
nutrition. 

Insectivorous   plants.  —  Besides    the   ordinary   parasites   and   sapro- 
phytes, there  are  a  few  rather  isolated  cases  of  green  seed  plants  which 


Fig.   657.  —  A   rosette   of   leaves   of   Venus's    flytrap    (Dionaea   muscipula)  seen   from 
above.  —  From  a  photograph  by  G.  W.  Oliver. 

have  special  apparatus  for  capturing  small  animals  and  digesting  them. 
Some  are  submersed  water  plants,  some  grow  on  land.  They  are  col- 
lectively known  as  insectivorous  or  carnivorous  plants,  but  the  methods 
of  capture  are  quite  diverse. 

Pitcher  plants.  — The  pitcher  plants,  Sarracenia,  Darlingtonia,  Nepen- 
thes (fig.  656),  and  Cephalotus,  have  part  or  all  of  the  leaf  trumpet-like, 
pitcher-like,  or  cuplike,  holding  more  or  less  water.  The  sides  have  stiff 
downward-pointing  hairs,  slippery  areas  of  treacherous  footing,  decep- 


NUTRITION 


387 


tive  translucent  spots  away  from  the  concealed  opening,  one  or  all,  whi<  h 
prevent  the  escape  of  insects  that  wander  in  and  sooner  or  later  drown 
in  the  fluid;  whence  nitrogenous  compounds  derived  from  their  bodies 
by  decay  or  digestion  enter  the 
tissues  of  the  pitcher. 

Flytrap.  —  Venus's  flytrap 
(Dionaea,  fig.  657)  has  leaves 
with  two  terminal  lobes  about 
1  cm.  long,  hinged  about  the 
midrib,  and  fringed  with  long 
slender  teeth,  which  interlock 
when  the  lobes  shut  together  (figs. 
658,  659) .  On  the  surface  of  each 
lobe  are  three  large  sensitive  bris- 
tles, and  if  one  of  these  be  bent  so 
as  to  compress  the  basal  cell,  the 
lobes  shut  like  the  two  jaws  of  a 
trap.  Insects,  flying  or  crawling, 
which  come  into  contact  with  the 
bristles  are  often  caught.  Then 
the  glands  upon  the  upper  (in- 
ner) surface  pour  out  a  digestive 
fluid,  the  proteins  are  reduced 
to  such  simplicity  that  they  can 
enter  the  tissues,  and  after  a  few 
days  the  leaf  opens  again.  Its 
water  mate,  Aldrovanda,  has  a 
similar  but  smaller  trap,  by  which 
minute  swimming  crustaceans, 
Dapknia,  Cyclops,  etc.,  arc  often 
caught. 

Sundew.  —  Drosera,  the  sun- 
dew, has  its  leaves  (fig.  685) 
fringed  and 

stalked  glands  that  secrete 
viscid  transparent  fluid,  in  which  small  insects  alighting  may  become 
enveloped  by  their  own  struggles,  and  further  (in  our  species)  on  account 
of  the  inflection  of  the  >talks  of  the  glands.  When  an  insect  i-  caught, 
the  character  of  the  secretion  changes;    it  becomes  more  watery  and 


Figs.  658,  659.  —  ( tass  set  tions  of  the  termi- 
nal Id1.cs  forming  the  "trap"  of  Dionaea: 
658,  enlarged  view,  dosed  position,  diagram- 
matic; g, digestive  glands;  />,  ^parenchymatous 

tissues  whose   varying   turgor  opens  and   doses 

tin- "trap";  s,  sensitive  bristles;  659,  outline, 
»vered  above  with    on  a  smaller  scale,  of  same  in  an  open  position. 

\fter  Kn'V. 


388  PHYSIOLOGY 

contains  an  enzyme  which  digests  proteins.  That  the  products  enter 
the  plant  and  are  advantageous  has  been  shown  by  comparing  fed  and 
unfed  plants  in  the  same  pot.  Those  on  whose  leaves  tiny  bits  of  meat 
and  egg  were  placed  were  larger  and  thriftier,  and  had  more  flowers, 
as  well  as  more  and  larger  seed,  than  the  ones  which  grew  under  identi- 
cal conditions  without  feeding. 

The  capture  of  insects  probably  supplements  a  scanty  supply  of  nitro- 
gen obtained  from  the  soil  nitrates;  but  too  little  is  known  of  the  ecology 
of  such  plants  to  establish  this  explanation  as  at  all  conclusive. 

A  fuller  discussion  of  most  of  the  topics  of  this  chapter  will  be  found 
in  Part  III. 

5.    THE    STORAGE    AND   TRANSLOCATION    OF    FOOD 

Surplus  food.  —  A  part  of  the  food  made  by  a  plant  is  promptly  util- 
ized in  the  making  of  new  tissues  (growth)  and  in  the  repair  of  the  pro- 
toplasm which  has  undergone  changes  in  the  course  of  its  activity.  It 
is  often  said,  also,  that  a  part  of  it  is  oxidized  directly  to  furnish  energy 
for  growth  and  other  work;  but  it  is  at  least  doubtful  whether  this  is 
true.1  However  that  may  be,  most  plants,  at  least  at  some  period  of 
their  existence,  make  more  food  than  they  actually  use  at  the  time.  The 
surplus  is  then  stored  for  a  longer  or  shorter  time,  until  it  is  required. 
But  it  may  never  be  used. 

Storage  places.  — Accumulation  may  take  place  in  the  very  part 
where  the  food  is  made;  but  usually,  if  there  is  any  room  there,  it  is 
insufficient;  and  to  judge  from  the  infrequent  storage  in  food-forming 
organs  these  two  functions  are  not  fully  compatible.  So  when  there  is 
any  considerable  surplus  of  food,  it  migrates  to  some  more  or  less  spe- 
cialized storage  organ.  In  the  lower  plants  these  are  relatively  simple, 
for  ordinarily  such  plants  make  little  excess  food.  In  Marchantia, 
for  instance,  the  colorless  parenchyma  of  the  lower  part  of  the  thallus 
is  accounted  the  storage  region.  In  the  pteridophytes  and  sperma- 
tophytes,  any  one  of  the  larger  organs,  root,  stem,  or  leaf,  may  become 
the  seat  of  food  accumulation.  In  many  cases  there  is  marked  change 
in  structure  and  form. 

Parenchyma  increased.  —  The  characteristic  change  in  structure  con- 
sists of  an  exaggerated  development  of  parenchyma,  in  which  chiefly  the 

1  The  matter  wjll  be  discussed  further  in  the  section  on  Respiration,  p.  403. 


NUTRITION 


3»9 


food  accumulates.  This  may  l>c  the  parenchyma  of  the  cortex,  or  of 
the  vascular  bundles,  or  of  the  pith;  <>r  all  may  be  involved.  One  note- 
worthy point  is  that  the  storage  tissues  arc  composed  of  live  cells,  even 
though,  as  in  -nine  ferns,  they  arc  very  thick  walled.  It  is  to  be  observed 
also  that  the  reservoirs  of  food  are  usually  Located  in  part-  thai  persist 
through  a  dry  or  cold  season  unfavorable  to  growth,  and  that  have  rudi- 
mentary growing  points  capable  of  quick  and  vigorous  development 
by  using  the  adjacent  suqilus.  So  the  seeds, 
bulbs,  tubers,  rhizomes,  etc.,  are  organs  of 
propagation,  and  by  way  of  attaining  that  end 
become  also  organs  of  storage.  (See  Part  III 
on  seeds,  bulbs,  and  tubers.) 

Storage  cells  active.  —  The  storage  of  food 
is  not  merely  a  stuffing  of  passive  cells  with 
surplus  food;  it  involves  the  activity  of  the 
storage  cells  themselves,  at  least  for  the  ac- 
cumulation of  the  food,  and  usually  also  for 
the  mobilization  when  this  food  is  about  to 
travel  to  growing  regions  where  it  is  subse- 
quently used.  The  process  of  mobilization  is 
commonly  called  digestion  (see  p.  397),  and  J^ JSSirfKS 
seems  to  be   the  reverse  of  the  process  by    onia:  660,  simple  starch  grain 

which    the   storage   forms  of  food  are  pro-    £*  '^^'f   in  ***».! 
&  »  661,  leucoplast  alone  of  asimi- 

duced.  lar  grain;   662,   leucoplast  of 

■After 


The  storage  forms  of  food    a  Uv,n  sram-    x  9°°-- 
Meykr. 


Storage  forms. 
are  chiefly  starches,  sugars,  hemi-celluloses, 
inulin,  fats,  and  proteins.  From  this  list  it  will  be  apparent  that  carbo- 
hydrates predominate,  and  quantitatively  they  form  much  the  greater 
part  of  stored  food. 

Starches.  —  Starches  are  stored  in  the  form  of  grains,  many  having  a 
form  characteristic  of  the  plant  in  which  they  are  found.  The  grains  are 
organized  by  the  activity  of  cell  organs  called  leucoplasts  or  amyloplasts 
(figs.  600-062),  which  seem  to  take  the  material  as  it  comes  to  the  cells, 
perhaps  as  glucose,  and  combine  it  into  larger  and  more  complex  mole- 
cules, that  finally  become  Stan  h.  This  is  disposed  in  the  interior  of  the 
leucoplast  as  one  or  more  grains,  which  at  length  stretch  it  enormously, 
or  even  rupture  it.  The  actual  structure  of  the  grain  is  believed  to  be 
that  of  a  spherite;  thai  is,  il  is  composed  of  a  multitude  of  mil  roscopi- 
cally  minute,  threadlike  crystals,  radiating  from  its  organic  center.      If 


390  PHYSIOLOGY 

more  than  one  such  crystal  starts  in  the  leucoplast,  a  compound  or  aggre- 
gate grain  may  result  (fig.  662).  The  grains  may  show  irregular  layers 
(fig.  660),  this  appearance  signifying  differences  in  the  proportion  of 
water,  composition  of  material,  etc.,  doubtless  determined  by  variations 
in  the  available  sugars  and  other  conditions  during  the  growth  of  the 
grain. 

The  starchy  reservoirs  are  sources  of  important  foods  for  men  and  animals,  as 
well  as  plants.  Many  of  our  farm  and  garden  crops  are  such  storage  organs, 
greatly  improved  and  enlarged  by  breeding.  Potatoes,  sweet  potatoes,  yams,  all 
the  cereals,  peas  and  beans,  arrowroot,  sago,  and  tapioca  are  widely  used  plant 
products,  whose  most  abundant  constituent  is  starch.  The  extraction  of  starch  for 
commercial  purposes,  especially  from  potatoes  and  corn,  is  an  industry  of  consider- 
able magnitude,  as  is  also  the  production  of  alcohol  by  the  fermentation  of  glucose 
derived  from  the  starch  of  these  plants.  The  following  table  shows  the  approxi- 
mate starch  content  of  some  common  food  reservoirs,  in  percentages  of  their  dry 
weight. 

In  seeds  of  rice  ....  68  In  seeds  of  navy  beans  .  45 
In  seeds  of  wheat  ....  68  In  seeds  of  flax  ....  23 
In  seeds  of  corn  ....  60  In  seeds  of  almond  ...  8 
In  seeds  of  pea      ....     52        In  tuber  of  potato     ...     80 

Sugars.  —  The  chief  storage  form  of  the  sugars  is  saccharose,  or  cane 
sugar.  While  glucose  and  fructose  may  be  counted  as  constituents  of 
almost  every  active  cell,  they  do  not  accumulate  in  nature  to  any  great 
extent,  whereas  saccharose  in  some  plants,  such  as  sugar  cane  and  beet, 
is  almost  the  only  form  of  surplus  food,  and  in  many  it  accompanies  the 
reserves  of  starch.  The  commercial  supply  of  sugar  is  obtained  chiefly 
from  cane  and  beet,  while  sorghum,  maple,  and  certain  palms  furnish 
a  relatively  small  or  local  supply. 

Sugar  is  extracted  from  cane  by  crushing  and  washing,  clarifying  the  liquor 
and  concentrating  it.  Beets  are  finely  sliced  and  the  sugar  is  extracted  by  diffusion, 
then  recovered  by  clarification  and  concentration  of  the  solution.  The  cultivated 
races  of  beet  now  average  nearly  15  per  cent  of  sugar,  with  some  samples  going 
over  20  per  cent,  as  against  less  than  7  per  cent  when  breeding  began.  Cane  juice 
yields  10-18  per  cent,  and  maple  sap  2-5  per  cent  of  saccharose.  The  refining  of 
sugar  by  redissolving  and  purifying  removes  the  coloring  and  flavoring  matters 
which  give  to  crude  sugars  from  different  plants  their  distinctive  taste. 

"  Reserve  cellulose."  —  This  name  has  been  applied  to  food  accumu- 
lated upon  the  walls  of  cells;  yet  the  substances  are  quite  different  from  the 
cellulose  which  forms  the  permanent  part  of  the  wall,  and  should  rather 
be  called  hemi-celluloses.  They  consist  often  of  mannans  and  galactans, 
which  on  digestion  yield  mannose  and  galactose,  sugars  that  are  quickly 


M  TKITION  391 

transformed  into  other  compounds.  The  hemi-celluloses  art-  especially 
common  in  the  endosperm  of  seeds,  and  are  used  as  food  by  the  embryo 
in  germination.  They  are  deposited  in  layers  on  the  interior  of  the  cell 
wall-,  sometimes  to  the  greal  reduction  of  the  lumen;  yet  through  the 
pits  in  the  thickened  walls  the  protoplast  in  each  chamber  maintains 
communication  by  slender  threads  with  its  neighbor.  This  excessive 
thickening  imparts  to  such  seeds  a  hornlike  toughness,  as  in  the  coffee 
"bean,"  or  even  a  bony  hardness,  as  in  the  date  "stones."  Sometimes 
cotyledons  and  even  bud  scales  have  like  deposits  on  their  cell  walls. 

Inulin.  —  Inulin  is  comparatively  restricted,  being  characteristic  of 
a  few  large  families  (and  occasional  elsewhere).  It  occurs  dissolved  in 
the  cell  sip.  especially  of  subterranean  organs.  It  is  a  very  complex 
carbohydrate,  though  less  so  than  starch,  having  a  formula  w(C6H10O5), 
where  n  is  probably  as  much  as  12  or  18.  Whereas  starch  is  built  from 
glucose  units,  inulin  is  formed  by  the  condensation  of  fructose  units, 
and  is  comparable  in  complexity  with  some  of  the  dextrins,  which  starch 
yields  by  digestion.  When  inulin-containing  tissues  are  put  into  strong 
alcohol,  the  inulin  is  deposited  as  spherites  (see  Part  III,  fig.  1209). 

Fats.  —  Fats  are  among  the  most  important  and  valuable  of  surplus 
foods.  In  most  plants  they  exist  as  small  drops  of  oil  in  the  protoplast; 
but  in  some  cases,  as  in  cacao,  they  are  solid  at  ordinary  temperatures. 
The  most  universal  storage  place  for  fats  is  the  seed,  where  it  is  in  some 
cases  the  dominant  form  of  food,  and  in  almost  all  it  is  present  in  greater 
or  less  quantity.  It  is  by  no  means  confined  to  seeds,  but  occurs  in  the 
flesh  of  fruits  (olive),  in  rhizomes  (potato,  iris,  and  sedges),  in  bulbs 
(onion),  and  in  roots  (carrot).  In  almost  every  part  of  a  plant,  indeed, 
small  quantities  of  oil  may  be  found,  and  from  many  reservoirs  it  can  be 
extracted  in  commercial  quantities. 

True  oils  must  be  distinguished  from  volatile  or  essential  oils,  which  are  common 
in  leaves  and  flower  parts.  The  latter  usually  have  a  distinct  odor  and  make  a 
temporary  translucent  spot  on  writing  paper,  whereas  that  made-  by  true  oils  is 
lasting. 

Accumulated  oils  are  obtained  for  commercial  uses  by  crushing  and  pressure; 
lint  as  only  a  portion  of  the  oil  i  whi<  h  forms  2  to  68  per  c  ent  of  the  dry  weight)  >  an 
l.c  recovered  thus,  the  "  cake  "  remaining,  with  its  residue  of  oil  and  other  sub- 
may  still  be  valuable  food  for  animals,  as  is  the  case  with  cotton  and  flax 

seed. 

Proteins.  -  Proteins,  unless  they  take  on  a  specific  solid  form,  cannot 
readily  be  distinguished  from  resting  protoplasm.    Thus,  the  "  gluten  " 


392 


PHYSIOLOGY 


of  wheat  is  apparently  a  part 
of  the  network  of  protoplasm 
in  which  the  starch  grains  are 
imbedded.  The  best  known 
storage  forms  appear  in  vacu- 
oles of  the  endosperm  in  seeds. 
The  proteins  accumulate  in  the 
small  vacuoles,  and  upon  the 
loss  of  water,  characteristic  of 
maturation  for  a  resting  period, 
become  more  and  more  con- 
centrated, until  finally  they 
solidify,  forming  the  "  aleu- 
rone  "  or  protein  grains.  These 
Fig.  663.  — Outer  portion  of  a  cross  section    are  very  commonly  associated 


'  m 


QO't 


of  a  wheat  grain:  h,  various  integuments  of  the 
ovary  and  seed,  forming  the  husk;  a,  cells  of 
"aleuronc  layer"  of  endosperm,  loaded  with 
protein  grains;  b,  starch-bearing  cells.  —  After 
Cobb. 


with  reserve  starch,  either  in 
the  same  cells,  as  in  the  pea 
and  bean,  or  the  protein  grains 
are  characteristic  of  certain 
cells,  as  in  wheat  and  other  cereals,  where  they  abound  in  the  outer 
layer  of  the  endosperm  (fig.  663).  In  large  grains  some  proteins  may 
crystallize  out,  as  in  the  castor  bean  (fig.  664)  and  the  Brazil  nut,  but 
oftener  they  remain  apparently  homogeneous. 
Amides.  —  Amides  occur  in  such  quantities, 
especially  in  some  sappy  reservoirs,  that  they 
may  be  considered  as  stored  food.  There 
they  may  form  40-70  per  cent  of  the  nitroge- 
nous materials. 

Alkaloids.  — Some  recent  studies  of  cacao  ("cocoa  " ) 
and  coffee  make  it  probable  that  their  alkaloids  (see 
p.  415),  which  are  of  a  different  type  from  most,  may 
be  a  form  of  surplus  nitrogenous  food,  since  they  come 
again  into  use.  They  constitute  a  very  compact 
source  of  available  nitrogen. 

Combination  of  food.  —  It  must  not  be  sup- 
posed that  the  foods  above  named  accumulate 
independently.  On  the  contrary,  they  always 
occur  associated,  though  one  form  is  likely  to 
be  dominant.     Rarely,  if  ever,  are  they  so  re- 


FlG.  664.  —  Cell  from  en- 
dosperm of  castor  bean 
{Ricinus  communis'):  p,  />, 
protein  grains,  made  up 
of  amorphous  proteins,  crys- 
talline proteins  (c)  ("  erys- 
talloids")i  and  globular 
compounds  of -proteins  with 
calcium  and  magnesium,  the 
globoids  (g). — Adapted. 


NUTRITION  393 

lated  to  one  another  in  amount  as  to  form  what  animal  feeders  call 
a  balanced  ration.  Tin's  is  shown  by  the  fact  that,  when  growth  is 
resumed,  food  of  one  sort  is  not  used  in  the  ratio  which  it  hears 
to  others  stored  with  it.  often  indeed  the  reserves  are  not  exhausted 
until  the  plant  or  shoot,  having  begun  independent  manufai  ture,  is  able 
to  supplement  the  deficiencies  in  the  stored  ration.  Tims,  finally,  it 
may  utilize  all  the  accumulated  reserve,  hut  often  this  is  not  done,  and 
the  excess  is  again  stored  elsewhere 

Traveling  forms.  —  Since  the  plates  of  storage  are  seldom  the  places 
of  food  making  or  use,  translocation  of  food  usually  precedes  and  fol- 
lows storage.  Unfortunately,  little  is  known  about  the  translocation 
of  foods.  It  seems  clear  that  the  traveling  forms  must  he  relatively 
simpler  than  those  in  which  they  an'  stored.  Obviously,  they  can  travel 
only  in  solution,  and,  as  a  rule,  the  protoplasm  does  not  permit  the  pas- 
sage of  the  foods  in  their  storage  forms.  Thus,  cane  sugar  probably 
travels  as  glucose  and  fructose;  the  fats  a-  glycerin  and  fatty  acids; 
the  proteins  as  amides.  For  in  all  translocation  of  foods,  whether  in 
small  plants  or  large,  it  is  necessary  that  they  he  able  finally  to  diffuse 
through  live  cells,  and  the  more  complex  compounds  are  usually  un- 
ahle  to  do  this. 

Diffusion.  —  In  the  smaller  plants  osmotic  differences  alone  must 
account  for  the  transfer  from  cell  to  cell.  This  may  he  facilitated  by 
the  delicate  protoplasmic  connections  which  commonly  exist  and  would 
make  it  unnecessary  for  all  the  food  to  pass  through  the  coll  wall  itself. 
In  fungi  which  have  coenocytic  hyphae,  the  absence  of  transverse  parti- 
tions probably  facilitates  transfer;  while  the  surging  movements  that 
have  heen  observed  in  the  contents  of  certain  molds  (Mucorales)  would 
certainly  do  so.  Vet  actual  knowledge  regarding  the  translocation  of 
food  iii  even  the  simplest  plant  is  scant}-.  Food  obviously  get-  from 
place  to  plate,  and  there  is  apparently  no  way  for  it  to  do  so  except  by 
diffusion. 

Conducting  system.  —  In  the  larger  plants  a  conducting  system  is 
developed;  and  it  is  evidently  advantageous  that  the  -lower  movement 
of  diffusion  he-  supplemented  by  a  more  rapid  one  along  the-  chief  lines 
of  travel  when  the  factories  are  separated  by  considerable  distances  fn  m 
the  places  of  use  or  storage.  This  conducting  system  in  all  the  vascular 
plant-  consists  of  the  phloem  strands.  It  may  he  supplemented  in 
certain  large  families  by  the  latex  system,  though  the  fun<  ti<>n  of  the  latex 
is  somewhat  uncertain. 


394  PHYSIOLOGY 

Phloem  strands.  —  The  phloem  strands  are  usually  definitely  related 
to  the  xylem  strands  (which  carry  water),  though  they  occur  also  inde- 
pendent of  them.  In  most  seed  plants  there  is  a  phloem  strand  lying 
along  the  outer  face  of  a  xylem  strand,  and  except  in  the  monocotyledons 
there  is  generally  between  them  a  meristem  (cambium),  which  may  add 
to  the  radial  diameter  of  both  xylem  and  phloem.  It  may  also,  if  it 
extend  from  one  strand  to  another  around  the  axis,  produce  new  second- 
ary phloem  strands  between  the  old  ones.  The  phloem  strands  form 
a  continuous  system,  and  may  be  traced  from  the  stem  outward  into  the 
leaves  and  downward  into  the  roots.  So  followed,  they  usually  dis- 
appear before  the  xylem  strands  end;  that  is,  their  differentiation  does 
not  begin  so  early  in  the  rootlets  nor  extend  so  far  in  the  leaves. 

Elements  of  phloem.  —  The  elements  of  the  phloem  strands  are  sieve 
tubes,  companion  cells,  cambiform  cells,  and  parenchyma,  with  some- 
times mechanical  tissues,  though  the  latter  belong  more  commonly  to 
the  adjacent  tissue  systems.  It  is  impossible  to  specify  the  precise 
role  of  each  of  the  elements;  but  among  them  all  the  sieve  tubes  may 
be  considered  the  chief  lines  of  conduction,  the  others  being  supplemen- 
tary thereto.1  In  a  way  the  sieve  tubes  are  analogous  to  the  tracheae  of 
the  xylem;  particularly  in  that,  having  their  end  walls  partially  resorbed, 
they  constitute  tubes  through  which  the  foods  may  move  without  the 
delay  necessitated  by  osmotic  transfer  from  cell  to  cell. 

Evidence  of  conductivity.  —  The  reasons  for  assigning  conductive 
functions  to  the  phloem  strands  are  chiefly  these:  (i)  The  pith  is  so 
commonly  dead  and  its  cells  filled  with  gases  that  it  may  be  excluded  from 
consideration.  (2)  The  cortex,  too,  is  often  dead;  particularly  is  this 
almost  universally  true  of  the  older  parts  of  shrubs  and  trees  in  which 
it  is  frequently  sloughed  off  after  a  few  years;  yet  there  is  an  active  trans- 
fer of  foods.  Moreover,  the  movement  of  food  through  the  protoplasmic 
membranes  of  live  cells  is  apparently  too  slow  to  meet  the  needs  of  plant 
growth.  (3)  When  the  cortex  is  removed  by  surgical  operation,  the 
supply  of  food  seems  to  be  quite  adequate  to  permit  development;  but 
if  the  phloem  strands  are  interrupted,  transfer  of  foods  is  almost  or  quite 
stopped. 

This  is  particularly  noticeable  when  girdling  occurs  in  nature,  as  when  birds 
destroy  a  zone  of  bark  in  conifers  whose  wood  remains  able  to  conduct  water. 
The  tops  and  roots  (if  one  or  more  circles  of  branches  below  the  injury  remain, 

1  It  is  as  though  the  sieve  tubes  were  the  main  railway  lines  and  the  adjacent  tissue 
sidetracks  temporarily  occupied. 


NUTRITION 


395 


keeping    the  latter  supplied  with  food)  may  continue  t<>  live  for  years  (fig.  665), 
vet  the    vigorous  growth    is  above   the   injury.      Girdling  experiments  with    willow 

shoots  arc  often  cited  as  adequate  proofs  of  the  conductive  function  of  phloem. 
For  example,  by  removing  a  ring  of  cortex  g  mm.  wide, 
a  few  centimeters  from  the  lower  end  in  one  case  and 

several  times  as  far  in  another,  and  plat  ing  both  shoots 
in  water,  lateral  roots  and  slmots  develop  in  lx>th  cases. 
Their  vigor  is  somewhat  proportional  to  the  relative 
lengths  of  stem  below  and  above  the  girdling,  and  this  is 
taken  to  indicate  that  the  new  parts  can  draw  only  upon 
food  stored  in  the  part  of  the  stem  above  and  below  the 
girdling,  transfer  being  prevented  by  the  interruption  of 
the  phloerru  But  if  bridges  of  bark  be  left  across  the 
gap,  the  differences  of  development  tend  to  disappear; 
and  the  more  numerous  the  bridges  the  less  the  differ- 
ences. While  such  experiments  agree  fairly  well  with 
other  observations,  they  are  in  themselves  not  con- 
elusive,  since  the  results  are  complicated  with  obscure 
phenomena  of  regeneration,  and  perhaps  with  wound 
irritability. 

(4)  The  content  of  the  sieve  tubes,  which  is  a 
coagulable  slime,  consists  more  largely  of  foods 
than  would  be  at  all  likely  unless  the  sieve  tubes 
were  organs  of  either  conduction  or  storage,  and 
the  latter  supposition  is  unlikely  because  the 
foods  are  almost  entirely  in  solution.  In  atypi- 
cal case  analysis  showed  that,  excluding  water,        FlG-  665.  — Portion  of 

.  .  111  the  trunk  of  a  pine,  the 

the   constituents    were:     carbohydrates,    30    per    bark  completely  destroyed 
cent;  amides,  38  per  cent;  proteins,  20  per  cent.    hv  hirds  :lt  «•    A  single 

c-        -i  1         r        1    1  1      r      j  ui        11       circle  of   branches   below 

So  rich  a  supply  of   soluble  foods  could  hardly 


I!- 


be  found  anywhere  else.  (5)  A  bit  of  merely 
corroborative  evidence  is  derived  from  the  dis- 
tribution and  relative  development  of  the  phloem. 
No  plants  need  more  facile  movement  of  foods 


keeping  all  tin-  parts  lower 
than  a  scantily  supplied 
with  food,  the  upper  part 
made  .111  excessive  growth, 

especially  in  the  neighbor- 
hood of  the  wound,  but 
food    could    not    pass    a 


than   vines,  whose  stems  are  necessarily  slender   freely  (perhaps  not  at  all) 
and    long,  a.id   in   none  is  there  better  develop-    Original  in  the  museum  of 

'        I'urduel  niversitv. — rrom 

ment  of  the  phloem.  Indeed,  when  the  anatomist  photograph  supplied  bj 
wishes  to  study  the  largesl  and  most  specialized  Stanley  Coulter. 
sieve  tubes,  vine-  are  almost  invariably  selected.  Moreover,  where  the 
requirements  for  food  transfer  are  the  greatest,  a-  in  flower  clusters  and 
in  tlie  brain  lies  of  inflorescences,  the  phloem  strands  are  particularly 
well  developed, 


396  PHYSIOLOGY 

Rhythmic  translocation.  —  Since  leaves  are  the  principal  regions  of 
food  making,  which  is  distinctly  rhythmic  by  reason  of  the  alternation 
of  light  and  darkness,  the  translocation  of  food  shows  a  corresponding 
rhythm.  The  transfer  of  any  soluble  food  is  continuous,  and  the  rate 
is  determined  by  the  usual  factors  ;  but,  as  the  transportation  facilities 
are  overtaxed  during  the  day,  there  is  on  the  whole  an  accumulation  of 
food  in  the  leaves  then;  only  after  the  nightly  slackening  does  emptying 
of  the  leaf  become  obvious. 

That  a  leaf  which  shows  starch  near  the  close  of  a  day  may  show  none  in  the 
early  morning  does  not  necessarily  indicate  that  carbohydrates  have  been  carried 
off  during  the  night,  though  they  doubtless  are,  but  only  that  they  have  been  re- 
duced in  amount  in  some  way,  probably  by  migration  and  by  conversion  into  other 
foods. 

Causes  of  movement.  —  Nothing  is  satisfactorily  known  as  to  the 
causes  of  movement  in  the  phloem.  In  the  sieve  tubes  the  absence  of 
protoplasmic  membranes  closing  the  ends  surely  permits  more  rapid 
diffusion,  which  may  be  further  facilitated  by  mechanical  mixing  due  to 
bending  and  other  compression  of  parts  of  the  system.  That  the  con- 
tents are  under  pressure  is  shown  by  the  rapid  oozing  of  material  from  cut 
sieve  tubes,  an  amount  being  reported  in  Cucurbita  which  indicates  that 
one  or  even  two  internodes  had  been  emptied,  and  so  the  material  must 
have  passed  75  to  100  of  the  sieve  plates  (the  perforate  end  walls  of  the 
sieve  cells).  The  source  of  this  pressure  and  the  effect  of  it  on  translo- 
cation is  not  known. 

Latex  system.  —  In  certain  families,1  it  may  be  that  translocation  of 
foods  takes  place  through  the  latex  vessels,  as  well  as  by  the  phloem. 
Latex  vessels  form  a  system  of  branched  or  anastomosing  tubes  run- 
ning through  the  cortex  (more  rarely  elsewhere),  and  ending  blindly 
in  the  leaves  and  roots.  Histologically,  they  are  coenocytes  or  cell 
fusions  (see  Part  I,  p.  27).  They  approach  very  near  to  the  growing 
points,  and  in  the  leaves  have  close  relations  with  the  manufacturing 
cells,  the  very  arrangement  sometimes  suggesting  its  fitness  for  collect- 
ing foods.  The  latex  which  fills  these  tubes  is  the  cell  sap  of  a  huge 
vacuole,  the  protoplasmic  contents  being  reduced  to  a  very  thin  layer. 
Latex  is  in  part  a  watery  solution  of  many  substances,  such  as  proteins, 
sugars,  gums,  tannins,  alkaloids,  and  salts;  in  part  an  emulsion  of  oils 
and  tannins  in  droplets;  and  in  part  suspended  granules  of  starch,  gum, 

1  Particularly  the  Papaveraceae,  Compositae  (Cichorieae),  Lobeliaceae,  Campanu- 
la! i  .11  ,  Asclepiadaceae,  Apocynaceae,  Euphorbiaceae,  Moraceae,  Araceae,  and  Musaceae. 


NUTRITION  397 

resin,  and  caoutchouc.  Some  latex  is  translucent,  but  usually  it  is  an 
opaque,  white,  yellow,  or  orange  liquid,  Familiar  to  many  as  the  milky 
"  juice  "  of  dandelion,  poppy,  milkweed,  or  the  orange  "  blood  "  of  the 
bloodroot.  Latex  is  commercially  important  as  the  source  of  opium 
and  its  alkaloids,  of  India  rubber,  and  of  gutta  peri  ha. 

Function.  —  The  principal  reasons  for  ascribing  to  latex  vessels  the 
function  of  a  conducting  system  are  the  abundance  <>f  foods  in  the  latex, 
and  the  peculiar  structural  relations  of  the  latex  vessels  t<>  the  nutritive 
cells  of  the  leaves.  The  carbohydrate  and  nitrogenous  foods  of  the  latex 
run  as  high  as  30  per  cent  of  the  dry  matter  therein;  they  are  most  abun- 
dant when  active  growth  and  development  are  beginning,  and  least 
so  when  growth  is  checked  and  a  resting  period  i-  at  hand.  In  some 
leaves  the  latex  vessels  look  as  though  they  were  favorably  arranged  to 
receive  materials  collected  from  the  nutritive  cells.  Yet  for  the  conduc- 
tive function  the  evidence  is  rather  presumptive  than  convincing.  It 
may  be  that  the  latex  has  to  do  rather  with  storage  and  protection. 

For  further  details  on  latex  and  accumulation  of  foods,  see  Part  III. 

6.    DIGESTION 

Nature  of  digestion.  —  Whenever  foods  are  insoluble  in  water  (as  are 
some  of  the  mosl  valuable  ones),  they  cannot  be  used  by  plants  until 
transformed  into  a  soluble  substance.  Whenever  soluble  foods  are  un- 
able to  diffuse  readily  through  protoplasmic  membranes,  they  (an 
scarcely  mow  from  one  point  to  another,  and  are  available,  if  at  all, 
chiefly  in  the  cell  where  they  happen  to  be.  Every  transformation  of 
food  by  the  agency  of  a  third  body  from  an  insoluble  to  a  soluble  and 
from  an  indiffusible  to  a  diffusible  condition,  whatever  the  precise 
chemical  nature  of  the  change,  is  summed  up  in  the  term  digestion. 
This  use  of  the  term  is  in  exact  accord  with  its  long  use  in  animal 
physiology.  The  pn><  e— es  in  plant  and  animal,  indeed,  are  essentially 
the-  same;  they  are  wroughl  by  the  same  sorts  of  agents,  affect  the  same 
orts  of  substances,  and  result  in  the  same  sorts  of  products. 

No  special  digestive  organs.  —  Plants  differ  from  the'  larger  animals 
in  having  110  pou<  hed  tube  wherein  food  is  lodged,  and  in  which  some  of 
the  more  striking  digestive  processes  take  place,  before  the  food  truly 
enters  the-  body.  This  digestive  trait,  its  parts  and  accompanying 
gland  ,  constitute  the  special  digestive  organs  of  the  animal,  though 
mm  h  important  digestion  takes  phu  e  elsewhere.     Plants  have  no  spe<  ial 


398  PHYSIOLOGY 

digestive  organs  comparable  to  these;  but  places  of  food  making  and  food 
storage  must  be  places  where  digestion  is  also  particularly  active. 

Misleading  comparisons  of  the  leaves  to  the  stomach  not  rarely  occur  in  primary 
books,  which  thus  seek  to  "  explain  "  the  work  of  a  leaf.  When,  as  in  one  notable 
instance,  a  leaf  is  compared  to  a  kitchen,  where  the  dilute  "  soups,"  coming  up  from 
the  roots,  are  "  boiled  down";  later  to  a  stomach,  where  the  food  is  made  ready; 
and  finally  to  the  lungs,  by  which  the  dear  little  plant  breathes,  the  child  would  have 
a  truly  appalling  notion  of  a  leaf  were  he  not  usually  immune  to  such  bad  pedagogy, 
by  reason  of  his  ignorance  of  at  least  the  stomach  and  lungs. 

Extra-cellular  digestion.  —  In  plant  as  in  animal,  many  foods  must  be 
digested  before  they  can  enter  the  cells  at  all,  while  others  are  digested 
as  they  lie  in  the  cells.  So  one  may  distinguish,  as  to  location,  extra- 
cellular and  intra-cellular  digestion;  but  agents,  processes,  and  results 
are  essentially  alike  in  both.  In  a  fungus  which  merely  pushes  its  way 
among  the  intercellular  spaces  of  another  plant,  it  is  impossible  to  say 
whether  any  food  is  being  digested  or  whether  only  what  is  already 
soluble  and  diffusible  is  being  used.  But  when  a  fungus  sends  a  branch, 
as  a  haustorium,  through  the  cell  wall  (fig.  651),  or  when,  as  in  certain 
wood-destroying  fungi,  the  mycelium  penetrates  the  walls  freely  in  all 
directions,  it  is  obvious  that  by  some  means  the  wall  is  actively  dissolved 
at  the  point  of  contact. 

Chemical  changes.  —  The  changes  characteristic  of  digestion  result 
in  the  cleaving  of  compounds  into  two  or  more  simpler  substances,  with 
or  without  the  taking  up  of  water.  In  case  water  is  incorporated  the 
cleavage  is  called  hydrolysis. 

Thus  when  cane  sugar  is  digested: 

Ci2H22Oii    +    H20    ^>  C6H12Oe    +    C6H1206 
saccharose  water  glucose  fructose 

Starch  when  digested  takes  up  water,  and  four  fifths  of  it  breaks  up  into  maltose 
units  (Ci2H22On),  the  other  fifth  resisting  full  digestion  for  a  longtime.  The  mal- 
tose is  further  digested  into  two  units  of  glucose,  with  assumption  of  another  mole- 
cule of  water.  Other  foods  split  up  into  simpler  compounds  without  adding 
anything  to  their  members.  Thus  sinigrin,  a  glucoside  characteristic  of  the  plants 
in  the  mustard  family,  cleaves  thus: 


Ci0H18NKS2Oio  ^t    C3H6CNS     + 

C6Hi2Oc  +       KHSO4 

sinigrin                      allyl  thiocyanate 

glucose         potassium-hydrogen 

(mustard  oil) 

sulfate 

The  chemical  changes  of  digestion  represent  only  a  few  of  the  mul- 
titudinous reactions  going  on  in  the  plant.  The  rate  of  these  reactions, 
like  all  others,  depends  on  temperature,  concentration,  etc.,  and  espe- 


NUTRITION  399 

i  iallyon  the  effect  of  other  substances  whi«  li  arc  present.  It  is  not  always 
evident  just  how  a  third  body  affects  the  rate  at  which  one  substance  is 
converted  into  another  in  a  chemical  reaction,  and  so  doubtless  many 

effects  of  this  sort  pass  unnoticed.  Hut  when  the  effect  i>  pronounced, 
the  third  body  is  spoken  of  as  a  catalyst,  and  the  effect  of  the  catalyst 
on  the  reaction  is  known  as  catalysis.  By  such  agents  reaction-,  so 
slow  as  to  be  unnoticed,  may  be  greatly  accelerated  and  become  evident; 
and  others,  which  might  be  very  rapid,  arc  retarded,  even  until  they  are 
negligible. 

Enzymes.  —  Among  the  catalytic  agents  (which  arc  varied  and  not  at 
all  confined  to  living  beings)  are  certain  substances  produced  by  organ- 
isms and  called  enzymes.  These  are  widely  different  in  their  action, 
though  they  all  seem  to  be  of  protein  nature,  so  far  as  their  chemical  <  har- 
acter  is  made  out.  The  great  difficulty  in  doing  this  lies  in  the  impossi- 
bility, up  to  date,  of  separating  them  from  the  other  protein-  of  the  cell 
and  obtaining  them  in  any  certain  state  of  purity.  In  general  they  act 
best  within  certain  narrow-  limits  of  temperature,  such  as  30-450  C, 
and  most  are  totally  destroyed  at  such  temperatures  as  60-750  C.  Small 
quantities  of  free  acid  or  alkali  may  facilitate  their  action;  while  certain 
metallic  ions,  e.g.  Hg,  Cu,  Ag,  may  retard  or  inhibit  their  ordinary 
effect,  just  as  they  "  poison  "  a  live  cell. 

There  seems  to  be  a  great  variety  of  enzymes,  each  producing  an  ap- 
propriate effect  upon  certain  foods;  but  others  are  known  which  have 
to  do  with  reactions  quite  apart  from  the  digestive  changes.  The  di- 
gestive enzymes,  then,  are  only  part  of  a  larger  class  of  bodies,  whose 
number  and  variety  are  only  imperfectly  known. 

Reversible  action.  —  The  action  of  a  number  of  enzymes  is  known  to 
be  reversible;  i.e.  they  not  only,  under  certain  conditions,  hasten  the 
otherwise  imperceptible  decomposition  of  a  particular  substance  into 
two  or  more  simpler  compounds,  but  also,  under  other  condition-,  ac- 
celerate the  combination  of  the  simpler  substances  into  the  more  com- 
plex one.  Indeed,  it  seems  likely  that  the  constructive  action  of  enzymes 
may  soon  be  shown  to  be  as  important  as  the  destructive.  This  action 
would  be  of  the  greatest  importance  in  the  making  of  complex  food-  from 
simpler  ones,  such  as  the  formation  of  Starch  from  glucose,  of  1  ane  sugar 
from  glucose  and  fructose,  of  proteins  from  amido-compounds,  etc. 
But  the  knowledge  of  this  constructive  action  is  yet  very  -canty. 

Carbohydrate  enzymes. —  Diastase  is  one  of  the  most  important  and 
widespread  enzymes.     It  is  found  in  practically  all  part-  of  plants,  but 


400 


PHYSIOLOGY 


especially  in  leaves  and  storage  organs.  It  partly  digests  starch  into 
maltose,  a  residue,  representing  about  20  per  cent  of  the  grain,  resisting 
its  action  for  a  long  time.  In  the  course  of  decomposition,  various 
dextrins  are  produced  by  successive  cleavage,  presently  becoming  simple 
enough  to  be  analyzed.  The  last  member  of  the  series  breaks  into  mal- 
tose and  isomaltose,  C1L.H220n.  There  are  at  least  two  forms  (possibly 
more),  secretion  diastase  and  translocation  diastase,  differing  in  the 
mode  of  dissolution  of  the  starch  grain.  The  former  erodes  the  surface 
irregularly,  whence  narrow  canals  penetrate  the  interior,  and  the  grain 
often  falls  into  fragments;  the  latter  corrodes  the  grain  almost  evenly, 
reducing  it  gradually  in  size  until  it  disappears. 

It  is  probable  th.it  what  is  here  called  diastase  consists  of  at  least  two  enzymes; 
amylase,  which  digests  starch  to  a  dextrin,  and  dextrinase,  which  breaks  the  dextrin 
into  maltose;  this,  maltase  (see  below)  cleaves  into  glucose. 

Inveriase,  in  like  manner,  can  hasten  the  hydrolysis  of  cane  sugar 
into  two  hexose  sugars,  glucose  and  fructose. 

Trehalose  and  several  other  enzymes  in  fungi  attack  trehalose  and  other 
sugars  peculiar  to  them,  and  digest  them  into  the  hexoses  of  which  they 
were  originally  built. 

Maltase,  an  enzyme  which  is  often  associated  with  diastase,  carries 
the  process  of  starch  digestion  further,  cleaving  each  maltose  molecule 
into  two  molecules  of  glucose. 

Inulase  likewise  attacks  inulin,  breaking  it  up  into  levulins  and  finally 
into  fructose.     Perhaps  there  is  here  also  more  than  one  enzyme  at  work. 

Cytase  is  responsible  for  digesting  hemi-celluloses  (chiefly  mannans 
and  galactans)  of  seeds,  while  enzymes  under  the  same  name,  but  prob- 
ably different,  have  been  found  in  wood-destroying  fungi,  and  have  been 
assumed  present  whenever  a  tissue  is  penetrated  by  a  hypha,  or  by  a 
more  massive  member,  as  in  the  sinking  of  the  foot  of  bryophytes  into  the 
gametophyte  (see  Part  I,  p.  108)  and  in  the  emergence  of  the  branches  of 
roots  through  the  cortex  (fig.  667;  see  also  Part  I,  p.  250,  and  fig.  558). 

Fat  enzymes.  —  Lipase,  perhaps  of  several  different  forms  and  so 
deserving  distinctive  names,  has  been  found  in  organs  where  fats  are 
present,  especially  in  seeds  and  many  fungi.  Lipase  breaks  up  fats  into 
their  components,  fatty  acids  and  glycerin,  which  are  then  readily  dif- 
fusible. 

Glucoside  enzymes.  — These  are  common,  setting  free  glucose  from  many  dif- 
ferent compounds.     Emulsin,  for  example,  breaks  amygdalin,  a  glucoside  common 


NUTRITION  401 

in  peach,  almond,  and  apple  Beeds,  into  hydrocyanic  acid,  glucose,  and  benzoic 
aldehyde,  thus; 


Ca>Hs7NOii    - 

1     HaO 

-> 

C7H4O 

I-      IICN    + 

2(C«HuC 

amygdalin 

water 

benzoic 
aldehyde 

hydro 

i>. mi.  .1.  id 

glucose 

The  so-called  "mustard  oil  "  is  produced,  along  with  glucose  and  two  other  1  (im- 
pounds (see  p.  398)  from  sinigrin,  a  glucoside  <  hara<  teristi  of  the  mustard  family. 
These  actions  are  very  rapid,  as  shown  by  the  formation  of  the  peculiar  flavor  or 
pungency  almost  as  soon  as  the  parts  are  crushed  by  the  teeth  and  the  enzyme 
thus  brought  into  contact  with  the  glucoside. 

Protein  enzymes.  —  Several  enzymes  are  known  which  digest  proteins. 
In  animals  their  digestion  proceeds  by  two  prominent  stages:  first,  the 
peptic  enzymes  (i.e.  those  like  pepsin  of  the  stomach)  convert  proteins 
into  peptones,  which  arc  soluble  and  diffusible;  second,  the  trypsin  of 
the  intestine  converts  proteins  and  peptones  alike  into  amino-acids  and 
other  compounds,  still  more  freely  soluble  and  diffusible.  At  first 
protein  digestion  in  plants  was  ascribed  to  peptic  enzymes;  later,  be- 
cause of  its  completeness,  it  was  referred  to  tryptic  enzymes  and  the 
presence  of  peptic  enzymes  was  denied.  Now,  however,  it  is  possible 
to  distinguish  the  two  classes  of  enzymes,  though  they  act  together  and 
carry  forward  the  processes  to  completion  without  a  pause  at  any  par- 
ticular stage  of  simplification. 

Inasmuch  as  the  proteins  are  not  prominent  among  surplus  foods,  it  might  seem 
at  first  sight  that  protein  digestion  was  unimportant  in  plants.  But  aside  from  the 
stored  food,  many  instances  where  such  digestion  must  occur  may  be  cited.  Thus, 
the  exhaustion  of  proteins  to  a  large  extent  from  the  foliage  of  annuals  as  the  seeds 
ripen  (e.g.  as  shown  in  cereals),  and  the  partial  recovery  of  proteins  from  leaves 
of  trees  before  their  fall,  presuppose  protein  digestion.  So,  also,  the  action  of  a 
plant  parasite  or  saprophyte  on  animal  bodies,  and  of  the  curious  pitchers  and 
traps  of  carnivorous  or  insectivorous  plants  involve  protein  digestion. 

Assimilation.  —  All  the  digestive  changes  are  preliminary  to  the  trans- 
location of  foods  from  places  of  manufacture  to  places  of  storage  or  use, 
or  from  places  of  storage  to  places  of  use.  And  before  foods  arc  of 
real  use  they  must  be  incorporated  into  the  living  substances  of  the 
body,1  which  grows  thereby.  This  final  step  in  the  chemical  progress 
of  foods,  by  which  they  become  a  pari  of  the  living  protoplasm,  is  known 

1  This  view  is  only  partly  shared  by  those  physiologists  who  believe  that  f<«»l  can  be 
"oxidized"  <lirec  tly  to  serve  as  a  souri  eof  <  nergy.  See  these)  lion  on  Respiration  (p.  403). 
For  them  the  Food  so  oxidized  is  no  more  incorporated  into  the  body  than  fuel  is  into  the 
furnace  in  which  it  is  hurnt. 


402  PHYSIOLOGY 

as  assimilation.  To  give  it  a  name  is  about  all  that  can  be  done  at  pres- 
ent, for  until  very  much  more  is  known  of  the  chemistry  of  proteins,  of 
which  protoplasm  chiefly  consists,  practically  nothing  can  be  known  of 
the  details  of  assimilation. 

Metabolism.  —  The  important  steps  in  nutrition  are  these  :  (i)  the 
making  of  carbohydrates  in  green  parts  properly  lighted  out  of  H.,C03; 
(2)  varied  modification  of  these  and  incorporation  of  nitrogen  (often  also 
sulfur  and  phosphorus)  from  mineral  salts  to  form  amides  and  finally 
proteins;  (3)  the  assimilation  of  proteins  into  protoplasm.  On  the  whole 
these  steps  are  upward;  the  material  becomes,  though  with  many 
fluctuations,  gradually  more  and  more  complex,  until  it  enters  upon  its 
final,  most  complex,  least  stable,  living  condition.  It  is  maintained  for 
a  time  at  the  high  level  as  living  stuff,  or  it  becomes  a  part  of  some  more 
permanent  portion  of  the  body,  like  the  cell  wall ;  or  it  is  broken  up  and 
reduced  gradually  to  simpler  compounds,  some  perhaps  to  be  rebuilt 
into  living  matter  again,  some  to  break  into  simpler  and  simpler  com- 
pounds and  to  leave  the  body  (e.g.  as  C02,  H2,  etc.). 

Metabolism  is  an  old  general  name  for  all  the  chemical  changes  in 
a  living  organism.  The  constructive  phases  of  nutrition  are  often 
summed  up  in  the  term  anabolism  or  constructive  metabolism;  the  de- 
structive phases  as  catabolism  or  destructive  metabolism.  In  the  former 
the  processes  tend  to  be  synthetic;  in  the  latter  analytic.  Having  con- 
sidered the  synthetic  processes,  the  analytic  ones  demand  attention  in  the 
next  chapter. 


CHAPTER    IV.  — DESTRUCTIVE   METABOLISM 
i.    RESPIRATION 

Respiratory  organs.  —  The  word  respiration,  or  its  English  equivalent, 
breathing,  suggests  at  once  the  currents  of  air  into  and  out  of  the  lungs, 
and  the  bodily  movements  that  cause  them.  The  reason  for  this  is  that 
so  much  attention  has  been  given  to  these  matters  in  human  physiology 
that  the  more  important  processes,  which  take  place  in  the  muscles  and 
live  tissues  generally,  have  been  almost  ignored.  This  is  emphasized 
by  the  fact  that  the  phrase  "  respiratory  organs  "  means  the  lung-  and 
the  air  passages  thereto,  while  the  blood,  which  is  an  equally  important 
adjunct  to  the  aeration  of  the  tissues,  is  not  usually  included.  But  air- 
passages,  lungs,  chest  wall,  diaphragm,  blood  vessels,  and  blood,  not  to 
mention  others,  are  all  necessary  organs.  The  fundamental  processes, 
however,  take  place  in  the  living  cells;  and  they  go  on  there,  for  a  time 
at  least,  whether  or  not,  by  accessory  mechanical  means;  the  oxygen  of 
the  air  is  supplied  and  the  waste  products  removed. 

Since  in  plants  the  accessory  organs  are  very  simple  indeed,  their 
structure  and  behavior  needs  little  consideration,  particularly  as  they  are 
at  the  same  time,  in  green  plants,  related  to  transpiration  and  to  photo- 
synthesis (see  aerating  system,  p.  318).  So  botanists  have  focused 
attention  upon  the  essential  processes  in  respiration.  This  difference  in 
emphasis  has  tended  to  obscure  the  fundamental  likeness  of  this  Function 
in  plants  and  animals.1 

Identical  in  plants  and  animals.  —  Excluding  the  processes  of  aeration, 
respiration  in  plants  and  animals  is  alike  in  all  essentials.  When  the 
likeness  of  the  living  matter  in  the  two  is  considered — a  likeness  SO 
great  that  neither  microscopic  observation  nor  analysis  can  distinguish 
them  by  structure,  behavior,  or  composition  —  the  fundamental  identity 
is  not  surprising.      Yet  popularly  it  is  widely  believed  that  the  re^pira 

1  It  has  been  proposed  to  retain  the  term  respiration  for  the  aerating  processes,  ami  t.> 
use  the  term  energesis  for  the  <  hemi<  al  .  hanges  iii  the  tissues,  whose  end  seems  t<>  be  the 

setting  free  of  energy.      It  remains  to  lie  seen  whether  or  not  this  distinction  is  .n  <  eptable 

or  important.    It  may  prove,  indeed,  that  the  release  of  energy  is  quite  incidental  to  other 
more  essential  processes. 

403 


404  PHYSIOLOGY 

tion  of  plants,  or  of  green  plants  at  least,  is  exactly  the  reverse  of 
that  of  animals.  This  misconception  is  clue  to  confusing  the  effect 
produced  upon  a  limited  volume  of  air  by  the  respiration  of  animals 
and  by  the  photosynthesis  of  plants,  two  processes  which  are  as  little 
comparable  in  their  results  as  are  walking  and  eating. 

Neither  gaseous  exchange  nor  combustion.  —  The  striking  change 
that  most  organisms  produce  in  the  air  of  a  limited  space  is  the  reduction 
in  the  amount  of  oxygen  and  the  increase  in  the  amount  of  carbon  dioxid. 
This  can  readily  be  demonstrated  by  putting  a  considerable  quantity 
of  germinating  seeds  or  opening  flowers  into  a  fruit  jar  and  sealing  it  for 
a  few  hours.  On  then  lowering  a  lighted  taper  into  the  jar,  the  flame 
will  be  extinguished;  and  a  cup  of  baryta  water  will  be  covered  quickly 
with  a  film  of  barium  carbonate.  This  has  led  to  a  superficial  concep- 
tion of  respiration,  current  in  text-books  and  encyclopedias,  as  an  ex- 
change of  the  gases,  oxygen  and  carbon  dioxid,  between  the  air  and 
the  organism.  Because  in  the  burning  of  wood  and  other  carbon  com- 
pounds oxygen  is  consumed  and  carbon  dioxid  is  produced,  respiration 
has  been  assumed  to  be  a  process  of  oxidation,  in  which  foods  undergo 
"  combustion  "  in  the  same  sense  as  the  fuel  in  a  furnace,  the  energy 
being  liberated  as  heat  and  in  other  forms,  when  the  carbon  of  the  com- 
pounds is  combined  with  the  oxygen  of  the  air.  One  striking  difference 
between  "  combustion  "  inside  an  organism  and  outside  is  that  the  former 
occurs  at  low  temperatures,  while  the  latter  takes  place  commonly  at 
high  temperatures.  To  escape  this  difficulty  the  term  "  physiological 
combustion  "  was  invented.  But  the  conception  of  respiration  as  an 
exchange  of  gases  accompanying  oxidation  of  carbonaceous  foods  is 
inadequate,  and  comparing  it  to  any  sort  of  combustion  is  more  mislead- 
ing than  helpful. 

Aerobic  and  anaerobic  respiration.  —  In  the  first  place,  though  or- 
dinarily oxygen  is  fixed,  oxygen  is  not  indispensable  to  respiration; 
and  in  the  second  place,  though  ordinarily  C02  is  evolved,  carbon  dioxid 
is  not  a  necessary  product  and  probably  in  no  case  does  the  02  combined 
with  the  C  come  directly  from  the  air.  That  being  so,  it  is  obvious  that 
the  above-mentioned  conceptions  as  to  respiration  cannot  be  valid. 
That  respiration  sometimes  goes  on  in  the  absence  of  free  oxygen,  makes 
it  necessary  to  distinguish  normal  or  aerobic  respiration  and  intramo- 
lecular or   anaerobic  respiration.1     Aerobic    respiration    proceeds    only 

1  Inasmuch  as  under  the  conditions  one  is  as  really  normal  as  the  other,  and  as  the 
term  intramolecular  expresses    an  interpretation  of  anaerobic  respiration  which  is  no 


DESTRUCTIVE   METABOLISM  405 

when  Ojis  presenl  insufficient  quantities,  and  among  the  end  products 
two,  COs  and  H^O,  arc  characteristic,  though  formed  in  very  variable 

quantities  in  proportion  to  the  Oa  taken  up.  Anaerobic  may  replace 
aerobic  respiration  in  any  organism  when  I  )s  is  cut  off,  and  may  proceed 
for  a  long  time;  bul  the  end  products  are  various  and  quite  different 
from  those  of  aerobic  respiration.  Among  them  are  <<>mmonly  ethyl 
alcohol  and  hydrogen,  and  less  C02.  Certain  minute  organisms  may 
pass  their  whole  existence  without  oxygen,  which  indeed  hinders  or  alto- 
gether stops  their  development,  and  they  are  thus  restricted  to  anaerobii 
respiration.  In  most  organisms,  however,  anaerobic  respiration  can 
be  considered  only  as  a  makeshift. 

Nature.  —  What  then  is  the  fundamental  feature  of  a  process  that 
goes  on  under  such  different  conditions  and  results  in  such  diverse  prod- 
ucts? So  far  as  now  appears,  respiration  consists  in  the  decomposition 
of  the  protoplasm  or  some  of  its  constituent  proteins,  either  directly, 
or  as  a  result  of  the  action  of  an  enzyme  or  of  some  internal  force  (stim- 
ulus) upon  it.  Inasmuch  as  the  inciting  cause  is  rarely  apparent,  spon- 
taneous or  self-decomposition  is  often  spoken  of,  but  this  merely  means 
that  the  reason  is  unknown. 

The  view  here  presentee!  is  not  the  one  most  generally  held  at  present,  hut  appeals 
to  the  author  as  most  consistent  with  the  known  fa<  ts.  Many  physiologists  consider 
respiration  to  consist  primarily  in  the  decomposition  of  foods  by  the  protoplasm 
or  l>v  enzymes,  without  their  assimilation  into  the  living  substance.  In  this  case 
f Is  arc  a  kind  of  fuel  for  the  body  (see  p.  406).  It  is  not  denied  that  some  de- 
composition of  protoplasm  <"<urs,  but  this  is  slight;  as  it  were,  a  sort  of  natural 
wear  and  tear  in  consequence  of  work. 

Advantage.  —  The  advantage  of  respiration  is  not  certainly  known, 
but  as  the  plant  in  order  to  do  work  must  expend  energy,  the  inferen<  e 
is  that  respiration  sets  free  energy  by  which  that  work  is  performed. 
Now  complex  and  unstable  compounds  contain  much  available  potential 
energy,  the  store  of  which  is  diminished  when  they  decompose,  and  the 
essence  of  nutritive  processes  is  the  building  up  of  those  compounds 
which  disappear  in  respiration.  Furthermore,  heat,  one  easily  observed 
form  of  energy,  is  generated  by  respiration,  though  it  is  not  known  that 
this  is  of  any  servi<  e  t<>  the  plant.  Hut  the  mosl  definite  reason  for  con- 
necting the  release  of  energy  with  respiration  Is  that  those  tissues  in 
whit  b  growth  or  other  work  is  proi  eeding  rapidly  arc-  also  i  harai  terized 

longer  tenable,  the  words  aerobic  and  anaerobic  (aer,  air;   bios,  life;   ■'",  not),  applied 

first  to  organisms  that  live  in  air  or  nourish  only  when  it  is  excluded,  are  preferable. 


4o6  PHYSIOLOGY 

by  rapid  respiration.  Thi.^  is  in  harmony  with  numberless  observa- 
tions in  animals,  in  which  the  work  can  be  increased  at  will,  when  a 
corresponding  increase  in  the  products  of  respiration,  the  consumption 
of  nutritive  materials,  and  the  evolution  of  heat  is  readily  shown.  It  is 
perhaps  better  to  consider  all  those  phenomena  of  respiration  as  its  re- 
sults, the  decomposition  of  the  protoplasm  being  the  primary  and  essen- 
tial feature.  Indeed  the  phenomena  of  respiration  may  all  be  directed 
to  ridding  the  body  of  the  products  of  an  inevitable  decomposition  of 
the  unstable  proteins  of  the  living  protoplasm. 

Role  of  oxygen.  —  When  energy  is  released  from  chemical  compounds, 
the  more  completely  they  are  decomposed  the  more  energy  is  liberated, 
as  a  rule.  In  anaerobic  respiration  the  decomposition  does  not  go  so  far 
as  in  aerobic,  for  the  resulting  substances  are  not  so  simple,  and  probably 
therefore  the  energy  released  is  far  less.  The  fact  that  growth  either  does 
not  occur  at  all,  or  is  very  limited,  when  oxygen  is  cut  off  from  plants 
accustomed  to  it,  also  indicates  this.  Herein,  indeed,  appears  the  prob- 
able role  of  oxygen  in  respiration.  It  seems  to  be  necessary  not  to  com- 
bine with  carbon  compounds,  but,  by  combining  with  and  so  removing 
substances  whose  presence  interferes  with  the  usual  reactions,  to  enable 
the  respiratory  processes  to  go  on  to  completion. 

The  common  idea  is  that  oxygen  combines  directly  with  carbon  and  so  causes 
"  combustion."  But  chemical  studies  of  the  combustion  of  certain  gases  show 
that  it  does  not  do  this,  even  at  high  temperatures.  Water  vapor,  which  yields  II 
and  OH  ions  by  dissociation,  furnishes  the  necessary  OH  ions  for  facilitating  the 
decomposition  of  the  carbon  compounds,  and  this  decomposition  does  not  proceed 
at  all  in  the  absence  of  water,  not  even  in  pure  oxygen.  The  oxygen  does  combine, 
however,  with  hydrogen  to  regenerate  water,  so  that  a  small  quantity  of  water  serves, 
provided  O2  is  continually  supplied.  In  this,  O2  behaves  somewhat  as  the  depolar- 
izer does  in  a  galvanic  battery,  wherein  its  function  is  that  of  an  oxidizing  agent  to 
convert  into  water  the  hydrogen  that  otherwise  would  accumulate  on  the  cathode 
and  stop  the  chemical  action.  Undoubtedly  other  "depolarizers"  than  oxygen 
are  present  in  the  cells ;  and  in  some  organisms  the  long  continuance  of  anaerobic 
respiration  without  serious  harm  may  be  thus  explicable.  The  presence  of  oxidiz- 
ing enzymes,  also,  may  be  essential  to  the  fixation  of  oxygen. 

End  products.  —  When,  therefore,  O  is  supplied,  the  end  products  of 
decomposition  are  in  large  part  the  most  stable  ones,  C02  and  H20. 
When  02  is  not  available,  these  are  less  prominent,  while  ethyl  and  higher 
alcohols,  organic  acids,  aromatic  compounds,  hydrogen,  etc.,  are  the 
more  abundant  end  products.  In  the  one  case  certain  parts  of  the  pro- 
toplasm break  into  simpler  and   simpler  compounds;  in  the  other  the 


DESTRUCTIVE   METABOLISM  407 

decomposition  stops  while  yet  the  materials  arc  complex,  and  hydrogen 
appears  because  do  oxygen  is  available  to  combine  with  it. 

Why  carbohydrates  disappear.- — The  end  products,  however,  prob- 
ably do  not  represent  in  any  case  the  whole  of  the  protein  molecule. 
Certain  fragments  of  it,  under  suitable  conditions,  go  down  into  COs 
and  H20;  but  others  are  not  so  far  split  up  that  they  cannot  be  rebuilt, 
with  necessary  additions,  into  protein  again.  It  seems  to  be  the  com- 
ponents of  the  protein  molecule  derived  from  carbohydrates,  which  are 
particularly  liable  to  complete  decomposition.  If  this  nucleus  alone  were 
broken  up,  the  ratio  of  free  Ol,  fixed  to  C02  produced  should  have  a 
value  of  unity.  This  is  not  by  any  means  true:  the  average  is  below  1 
and  the  value  varies  from  0.3  to  5.0;  so  it  is  probable  that  the  process 
is  complicated  by  the  interaction  of  other  substances.  The  repair 
of  the  proteins  requires  chiefly  carbohydrates,  because  the  nitrogenous 
losses  in  the  plant  are  quite  inconsiderable  as  compared  with  those  of 
an  animal.  So  a  marked  effect  of  respiration  is  a  disappearance  of  the 
accumulated  carbohydrates. 

The  assumption  that  carbohydrates  are  directly  decomposed  in  respiration  rests 
largely  on  the  fact  that  the  value  of  the  ratio  ( )2:C<  >a  is  affected  by  the  food  supplied 
to  non-green  plants.  Thus,  in  Aspergillus  it  ranges  from  0.4,^  with  10  per  cent 
tannin,  to  1.7S  with  10  per  cent  glucose,  indicating  that  not  composition  alone  but 
other  and  unknown  factors  are  concerned.  And  composition,  as  well  as  these  un- 
known factors,  may  produce  this  result  indirectly,  through  their  influence  on  assimi- 
lation, quite  as  effectively  as  by  directly  modifying  the  "  combustion  "  of  foods. 

Loss  of  weight.  — The  transformation  of  carbohydrates  in  the  repair 
of  proteins  can  have  little  effect  on  the  weight  of  the  plant;  but  the 
escape  of  C02  as  a  gas  and  the  evaporation  of  the  water  produced  does 
result  in  a  loss  of  weight.  If  the  total  dry  weight  of  seeds  be  calculated 
(the  percentage  of  water  in  like  seeds  having  previously  been  determined), 
and  these  seeds  be  grown  for  some  weeks  in  the  dark,  plants  of  consider- 
able size  can  be  raised.  But  on  drying  them,  the  residue  will  be  found 
to  weigh  less  than  the  calculated  dry  weight  of  the  original  seeds.  This 
difference  corresponds  to  the  combined  COs  and  H20  produced  and  lo>t 
in  the  course  of  respiration. 

Production  of  heat.  —  The  heat  produced  by  respiration  is  often  not 
observable  at  all,  unless  some  means  are  used  to  prevent  its  radiation  and 
its  transfer  to  the  air  by  the  evaporating  water,  [f  a  ma—  of  wheat 
seeds  be  germinated,  a  thermometer  thrust  into  the  mass  will  show  a 
temperature  considerably  higher  than  that  of  the  air  ;    but  this  is  duo 


408  PHYSIOLOGY 

largely  to  microorganisms,  whose  active  respiration,  and  especially  the 
fermentation  they  cause,  liberates  much  heat.1  If,  however,  the  surface 
of  the  seeds  is  carefully  sterilized  before  germinating,  the  difference  is 
much  less,  in  many  cases  with  ordinary  insulation  only  1-1.50  C. 

By  using  Dewar  flasks,  which  afford  very  perfect  protection  against  loss  of  heat 
by  radiation  and  conduction,  differences  of  200  C.  or  more  hare  lately  been  found 
with  80  gm.  of  peas  (weighed  dry). 

The  opening  of  flowers  crowded  into  a  compact  cluster  within  a  bract, 
as  in  the  calla,  causes  a  decided  rise  of  temperature,  differences  of  5-100  C. 
having  been  noted.  This  production  of  heat  is  continuous,  though  its 
rate  varies.  It  is  said  that  a  kilogram  of  seedlings  may  produce  heat 
enough  per  minute  to  warm  1  gm.  of  water  from  o  to  50  or  even  ioo°  C. 
Yet  under  ordinary  circumstances  this  heat  is  steadily  dissipated. 

Comparative  activity.  —  It  is  commonly  supposed  that  at  best  the 
aerobic  respiration  of  plants  is  weak  compared  with  that  in  animals. 
This  is  a  mistake.  The  respiratory  rate  for  active  tissues  of  plants 
compares  well,  weight  for  weight,  with  that  of  even  warm-blooded  ani- 
mals, and  in  some  cases  far  exceeds  it,  if  the  gaseous  changes  may  be 
taken  as  a  fair  measure  of  the  process.  Thus,  if  a  man  of  75  kg.  pro- 
duces at  light  work  about  900  gm.  C02  in  24  hours,  the  output  of  C02 
equals  1.2  per  cent  of  his  weight.  By  the  buds  of  lilac  the  output  of 
C02  equals  1.8  per  cent  of  their  weight;  by  those  of  horse  chestnut,  3 
per  cent;  by  seedlings  of  poppy,  2  per  cent;  by  molds,  6  per  cent.  While 
a  man  may  use  in  24  hours  1  gm.  of  oxygen  for  each  100  gm.  of  his 
weight,  young  leaves  of  wheat  use  it  at  the  same  rate;  opening  flowers 
use  4  times  as  much,  and  some  bacteria  200  times  as  much. 

The  stage  of  developmtnt,  the  general  activity,  and  the  rate  of  growth  influence 
decidedly  the  rate  of  respiration.  The  younger  and  more  active  the  tissues  or 
organs,  the  more  rapid,  as  a  rule,  is  the  respiration. 

Life.  —  It  has  already  been  indicated  that  anaerobic  respiration  begins 
like  aerobic,  but  that  the  decompositions  cease  before  they  attain  the 
same  extent.  It  may  very  well  be,  also,  that  they  pursue  a  somewhat 
different  course,  on  account  of  the  lack  of  oxygen.  Growth  ordinarily 
ceases  when  growing  tissues  are  forced  to  do  without  02,  though  some- 

1  When  moist  plants  or  manures  are  piled  up,  very  high  temperatures  may  he  produced 
in  the  midst  of  the  mass  by  the  combined  activities  of  many  different  fungi  and  bacteria. 
This  "heating"  may  even  suppress  or  kill  off  all  species  except  those  that  flourish  at 

55-65*  c. 


DESTRUCTIVE    METABOLISM  409 

times  it  continues  for  a  time;  whence  it  Is  inferred  that  the  energy  re- 
leased by  anaerobic  respiration  is  usually  inadequate  lor  growth.  Life, 
however,  persists  for  a  variable   time,  sometimes  for  weeks  or  months, 

though  in  active  parts  the  functions  are  much  disturbed  after  a  few  hours, 
and  death  shortly  ensues. 


2.    FERMENTATION 

Microorganisms.  —  The  fact  that  anaerobic  respiration  gives  rise, 
among  other  things,  to  alcohol  and  carbon  dioxid,  suggests  at  om  1 
relation  to  a  process  long  known  to  occur  in  sugary  juices,  like  those  of 
grapes  and  apples,  when  they  are  allowed  to  stand  unsterilized  and  un- 
sealed. The  sugar  disappears,  bubbles  of  gas  (COo)  rise  through  the 
liquid,  and  considerable  alcohol  is  formed  in  it.  This  process  is  known 
as  fermentation.  It  was  shown  long  ago  to  be  due  to  the  presence  of 
yeast  plants,  for  it  does  not  occur  when  they  are  excluded.  Further 
study  has  shown  that  analogous  changes  which  take  place  in  organic 
substances,  many  of  them  (like  the  souring  of  milk  and  the  spoiling  of 
meat)  being  familiarly  known,  are  due  to  the  action  of  other  micro- 
organisms. The  application  of  the  term  fermentation  has  now  been 
extended  to  cover  all  these  changes. 

Names.  —  Fermentations  are  named  after  the  most  prominent  or  de- 
sirable substance  produced,  or  sometimes  after  the  substance  destroyed. 
Thus,  the  fermentation  of  glucose  (grape  sugar)  is  alcoholic  fermenta- 
tion; that  of  lactose  (milk  sugar)  is  lactic  fermentation;  that  of  alcohol 
is  acetic  fermentation;  because  alcohol,  lactic  acid,  and  acetic  acid, 
respectively,  are  formed.  On  the  contrary,  the  cellulose  fermentation  is 
so  named  because  cellulose  is  destroyed.  When  proteins  are  attacked, 
evil-smelling  gases  are  among  the  products,  and  such  fermentations  are 
frequently  distinguished  as  putrefactions;  but  they  are  not  essentially 
different  from  others.  Only  a  few  of  the  better  known  and  more  impor- 
tant fermentations  can  be  treated  here. 

Alcoholic  fermentation.  —  The  alcoholic  fermentation  is  produced 
in  different  sugars  by  various  organisms.  The  sugars  that  arc  now 
known  to  be  fermentable  are  only  those  the  number  of  whose  carbon 
atoms  is  3  or  a  multiple  of  3;  thus,  the  trioses  <  11  I  '  .  hexoses 
(C6Hi206),  and  nonnoses  (C,H1809)  are  directly  atta<  ked;  while  the  more 
complex  carbohydrates  (di-  and  polysaccharides),  su<  h  as  cane  and  malt 
sugar  (CuHaOu)  and  starch  [5n(C8HioOj)],  are  fermented  .inly  after 


4io 


PHYSIOLOGY 


they  have  been  simplified  by  cleavage  into  hexoses.  Why  this  limitation 
exists,  and  why  within  this  there  are  others  even  more  specific,  is  not 
known.  The  organisms  concerned  are  chiefly  those  known  as  yeasts 
(see  Saccharomycetes,  p.  70),  but  certain  molds  and  bacteria  also  give 
rise  to  ethyl  alcohol,  though  the  latt  r  more  commonly  produce  higher 
alcohols  (propyl  alcohol,  butyl  alcohol,  etc.).  In  this  connection  it  is 
to  be  remembered  that  even  the  higher  plants  produce  ethyl  alcohol  in 
the  course  of  anaerobic  respiration. 

The  sugar  is  split  up  in  large  measure  into  C02  and  ethyl  alcohol, 
but  there  are  other  products,  such  as  glycerin,  succinic  acid,  etc.,  in 
smaller  quantity.  Fermentation  proceeds  very  slowly  when  the  yeasts 
are  abundantly  supplied  with  02;  then,  however,  they  grow  and  mul- 
tiply rapidly,  and  apparently  use  the  sugar  chiefly  as  food.  But  when 
the  supply  of  02  is  small,  so  that  their  vegetative  processes  are  hindered, 
fermentative  action  is  increased.  Though  alcohol  is  produced  at  all 
times,  its  quantity  is  in  a  sort  of  inverse  ratio  to  the  favorablencss  of  the 
conditions  for  life.  When  12  per  cent  have  accumulated  in  the  liquid, 
the  action  is  retarded,  and  by  14  per  cent  it  is  stopped. 

Fermentation  by  yeasts  was  long  believed  to  be  due  to  the  direct  action 
of  their  protoplasm  on  the  sugar;  now  it  has  been  proved  that  an  extract, 
made  by  grinding  the  yeast  with  sand  and  filtering  the  juice  under  high 
pressure  through  porcelain,  can  produce  the  same  effect.  The  active 
substance,  known  as  zymase,  is  soon  destroyed,  unless  protected  from 
digestion  by  accompanying  enzymes.  Similar  substances  have  been  iso- 
lated in  higher  plants,  which  are  believed  to  act  upon  carbohydrates  in 
anaerobic  respiration,1  giving  rise  to  alcohol  and  C02  in  the  same  propor- 
tions as  in  fermentation. 

The  economic  uses  of  alcoholic  fermentation  are  many.  It  plays  a  prominent 
rule  in  the  lightening  of  bread,  in  which,  however,  other  organisms  share  with  yeast 
the  production  of  the  gases  that  raise  the  dough;  it  is  the  source  of  commercial 
ethyl  alcohol,  which  is  distilled  from  fermented  liquids,  in  which  hexose  sugars  are 
first  produced  from  corn  and  potato  starch;  it  gives  rise  to  the  alcohol  in  a  host  of 
fermented  liquids  used  as  beverages :  wine,  beer,  koumiss,  pulque,  sake,  etc. 

Lactic  fermentation.  —  The  lactic  fermentation,  giving  rise  to  lactic 
acid,  is  best  known  in  the  souring  of  milk,  and  may  be  produced  whenever 
lactose  is  present  in  a  solution  to  which  the  lactic  acid  bacterium  has 

1  The  source  of  these  carbohydrates  is  uncertain.  They  may  be  either  the  unassimi- 
lated  carbohydrates  of  the  food;  or,  equally  well,  a  carbohydrate  nucleus  from  the 
decomposition  of  the  protoplasm. 


DESTRUCTIVE    METABOLISM  (i  i 

access.  As  in  the  alcoholic  fermentation,  the  accumulation  of  tne 
products  brings  the  action  to  a  standstill.  When  8  per  cenl  of  la.  i i . 
acid  has  accumulated  (or  less  in  milk),  the  bacterium  becomes  inactive. 

Acetic  fermentation.—  The  aceti<  fermentation  is  due  to  bacteria, 
which  oxidize  ethyl  and  other  alcohols  to  acids.  The  commonest  form 
converts  ethyl  alcohol  into  acetic  acid,  (  1 1 .  •  CI  I  .<  )I  I  +  (  ).^±  (I [8  • 
COOH  +  Il.O.  In  the  quick  process  for  the  manufacture  of  vinegar, 
in  which  this  fermentation  is  applied,  dilute  alcohol  (6-xo  per  cent)  is 
allowed  to  trickle  over  beech  shavings  in  a  deep  vat,  which  have  become 
covered  with  a  slimy  coating  of  the  organisms.  By  the  time  the  alcohol 
has  reached  the  bottom  it  has  been  oxidized  completely  to  at  eti<  acid. 

Butyric  fermentation.  —  Butyric  fermentation,  by  which  butyri<  acid 
is  produced  from  various  sugars,  especially  lactose,  and  indirectly  from 
polysaccharides,  through  the  agency  of  bacteria,  underlies  the  production 
of  desirable  flavors  in  butter  and  cheese. 

Putrefactions.  —  The  putrefaction  of  proteins  is  wrought  by  various 
bacteria,  but  little  is  known  of  the  details.  Among  the  numerous  end 
products  are  the  disagreeable  gases  hydrogen  sulfid,  mercaptans,  skatol, 
etc. 

So  a  multitude  of  fermentations  might  be  named,  each  concerned 
with  a  particular  compound  and  due  to  a  particular  organism.  By 
the  single  or  successive  action  of  such  organisms,  complex  organic  matter 
is  gradually  reduced  to  simple  forms,  like  those  from  which  it  was  con- 
structed, which  then  may  enter  again  into  the  cycle  and  be  built  up, 
through  the  agency  of  green  plants,  into  foods. 

Advantage.  —  The  precise  role  of  fermentations  in  the  life  history  of 
the  organism  that  produces  them  is  not  certainly  known.  It  is  possible 
that  they  are,  as  respiration  is  supposed  to  be,  a  source  of  energy.  The 
minuteness  of  the  organisms  would  make  possible  the  appropriation  of 
this  energy,  even  though,  in  contrast  to  that  set  free  by  respiration,  it  i- 
released  outside  the  body.  From  this  point  of  view  it  would  seem  that 
fermentation  might  be  considered  as  a  substitute  for  respiration,  though 
a  rather  ineffective  one.  and  hence  requiring  an  exaggerated  decom- 
position of  organic  matter.  On  the  other  hand,  it  has  been  suggested 
that  fermentation  serves  for  the  production  of  substances  in  which  the 
producers  can  live,  but  by  which  other  organisms  are  injured  and  so 
prevented  from  competing  with  them  for  food  and  room.  This  sugges- 
tion, however,  seems  forced  and  inadequate.     Yet  again,    it   may   be 


4I2  PHYSIOLOGY 

that  all  fermentations  are  effected  by  enzymes,  as  some  are  known  to 
be,  and  that  the  formation  of  these  enzymes  is  not  so  much  a  matter  of 
advantage  to  the  organism  as  an  inevitable  result  of  the  conditions  under 
which  it  develops.  If  tins  be  true,  to  seek  for  explanation  through  ad- 
vantage is  a  fruitless  quest. 


3.    WASTE    PRODUCTS    AND    ASH 

Wastes  not  useless.  —  In  the  course  of  the  many  and  varied  chemical 
changes  which  take  place  in  plants,  there  arise,  especially  in  consequence 
of  the  destructive  metabolism,  a  great  number  of  compounds  which  are 
not  usable  for  the  building  of  new  parts,  and  are  not  again  drawn  into 
the  metabolism.  Some  of  these  are  nevertheless  of  considerable  service 
to  the  plant,  and  in  varied  ways;  as,  for  example,  in  protecting  it  from 
predatory  animals  by  disagreeable  tastes  or  odors,  in  covering  wounds 
by  gummy  or  resinous  exudations,  in  attracting  by  color  or  odor  insects 
which  effect  pollination,  etc.  In  spite  of  the  usefulness  of  some  of  them, 
these  substances  are  often  called  waste  products,  and  this  word  may 
well  be  retained  instead  of  the  more  technical  term,  aplastic  products, 
which  has  been  applied  to  them.  For  in  every  household  there  are  like 
products,  properly  "  waste,"  so  far  as  the  direct  economy  is  concerned, 
some  of  which  may  nevertheless  be  collaterally  serviceable. 

Number.  —  Of  the  reactions  by  which  these  waste  products  are  pro- 
duced, not  much  is  known,  and  they  need  not  be  considered  at  all  here. 
The  number  of  the  products  is  very  great,  and  it  is  possible  to  name 
only  a  few  of  the  more  important  groups  and  examples  of  them.  An  im- 
pression of  their  number  may  be  gained  from  the  fact  that  in  a  recent 
work  on  plant  chemistry  more  than  4000  are  mentioned,  and  the  book 
does  not  pretend  to  enumerate  all  known  substances.  Thus  there  are 
over  200  known  alkaloids,  and  a  single  firm  lists  some  200  essential  oils 
of  commercial  value.  Yet  the  knowledge  of  the  chemistry  of  plants  is 
very  incomplete  and  lags  far  behind  that  of  animals. 

No  true  excretion.  —  Almost  all  of  the  wastes  accumulate  in  the  tis- 
sues, for  actual  excretion  by  plants  is  very  imperfect.  Except  for  those 
which  are  got  rid  of  in  the  fragments  of  bark,  roots,  twigs,  and  leaves 
that  are  shed,  and  the  relatively  minute  quantities  that  are  secreted  by 
surface  glands,  or  diffuse  out  into  the  water  from  roots  and  other  im- 
mersed parts,  there  is  no  provision  for  doing  more  than  storing  these 
substances  in  some  out-of-the-way  place.     In  no  case  is  there  any  ar- 


DESTRUCTIVE   METABOLISM  413 

rangement  for  continuous  riddance,  such  as  is  found  in  the  excretory 
organs  of  animals.  It  is  also  particularly  noteworthy  that  among  the 
wastes  there  are  few  or  none  except  the  alkaloids  that  contain  nitrogen. 
Even  these  are  not  necessary  products  of  metabolism,  for  the  very  plants 

that  produce  alkaloids  most  abundantly  may  be  so  grown,  and  healthily, 
as  not  to  contain  any. 

Gaseous  wastes. — Among  gaseous  wastes,  the  most  important,  C<  >_. 
and  02,  have  already  been  mentioned;  and  the  water  resulting  from 
respiration,  while  not  produced  as  a  gas,  leaves  the  body  mostly  in  this 
form.  In  a  few  plants,  notably  in  the  stinking  goosefoot  and  flowers  of 
hawthorns,  a  very  disagreeable  odor  makes  known  the  escape  of  a  gas, 
trimethylamin;  but  this  is  formed  only  in  trilling  amounts. 

Essential  oils.  —  Most  of  the  odors  of  plants,  fragrant  or  not,  are  due 
to  the  essential  (volatile)  oils,  which  are  distinguishable  from  true  oils, 
to  which  they  are  not  at  all  allied  chemically,  by  leaving  only  a  transient 
spot  on  paper.  They  are  especially  abundant  in  the  foliage  and  flowers, 
though  there  is  no  part  but  may  be  the  seat  of  their  production  or  storage. 
They  are  the  more  volatile  constituents  of  complex  mixtures,  secreted 
by  glands  of  various  forms  (see  p.  337),  whose  solid  residues,  after  the 
"  oils  "  have  been  driven  off,  are  resins  (see  below).  These  secretions 
may  escape  at  once  upon  the  surface,  or  they  may  be  stored  in  inter- 
cellular receptacles  and  released  only  by  crushing.  In  the  flower  leaves 
they  are  curiously  distributed,  being  formed  in  the  epidermis  of  both 
petals  and  sepals,  or  only  in  one,  or  only  in  the  cells  of  one  face,  or  only 
in  lines  or  patches  of  cells.  From  such  parts,  even  when  in  very  -mall 
amounts,  they  may  be  distilled,  and  when  more  abundant  they  may 
be  expressed  and  purified.  Some  are  medicinal,  and  some  an'  commer- 
cially valuable  as  perfumes  for  soaps,  ointments,  and  other  toilet  arti<  les. 
Chemically  they  are  quite  diverse;  many  of  their  constituents  belong 
to  the  class  of  compounds  known  as  terpenes. 

Gums  and  resins.- — Gums  and  resins  occur  in  great  variety,  and 
often  in  mixtures  called  gum-resins  and  balsams.  These  term-  are 
rather  loosely  used,  and  do  not  designate  definite  chemical  groups. 
The  true  gums  are  in  large  part  l  arbohyt Irate-,  arabinose  being  especially 
abundant  ((',-,! I,,,* )-),  and  arise  from  the  transformation  of  the  cell  wall 
and  growing  tissues  in  woody  plants.  They  swell  readily  in  water.  Gum 
arabic  and  gum  tragacanth  are  well  known  commercially,  and  the  gum 
of  cherry  and  peach  trees  is  familiar.  Resins  are  yellowish  solids, 
usually  derivatives  of  essential  oils,  that  occur  dissolved  in  essential  oils. 


414  PHYSIOLOGY 

Thus,  turpentine  consists  of  colophony  or  resin  dissolved  in  "oil  of  turpen- 
tine," itself  a  mixture  of  several  terpenes.  "  Canada  balsam,"  as  used  for 
mounting  sections,  consists  of  a  resin  solidified  by  driving  off  the  volatile 
oil  and  redissolved  in  a  more  volatile  solvent.  The  gum-resins  or  bal- 
sams are  variable  mixtures  of  gums  and  resins,  with  many  other  acci- 
dental constituents.  The  best  known  are  asafetida,  as  distinguished 
for  its  disagreeable  odor  as  are  galbanum,  myrrh,  and  frankincense, 
the  chief  components  of  incense  from  time  immemorial,  for  their  fragrant 
smoke.  They  exude  from  wounds  in  various  oriental  shrubs  and  solid- 
ify in  drops  and  irregular  masses. 

Organic  acids.  —  The  organic  acids  are  also  numerous,  but  four  pre- 
dominate. These  four,  oxalic,  malic,  tartaric,  and  citric  acids,  are  all  very 
widely  distributed  and  are  not  infrequently  associated.  Oxalic  acid 
(COOH  •  COOH)  is  not  certainly  known  to  occur  in  the  free  state,  but 
is  abundant  in  salts  of  calcium,  potassium-hydrogen,  and  magnesium. 
Calcium  oxalate  is  found  in  every  large  group  of  plants  except  bryo- 
phytes.  It  crystallizes  in  long  slender  needles  (raphides)  or  as  "  crystal 
sand,"  with  two  molecules  of  water ;  or  it  forms  large  single  crystals  or 
crystal  aggregates,  of  octahedral  form,  when  it  combines  with  six  mole- 
cules of  water.  (See  Part  III,  fig.  919.)  Magnesium  oxalate  forms 
spherites.  Malic  acid  (COOH  •  CH2  •  CHOH  •  COOH),  which  is  almost 
as  widely  distributed  as  oxalic,  occurs  in  the  juice  of  many  unripe  fruits, 
especially  the  apple,  pear,  cherry,  etc.,  either  free  or  in  salts  of  calcium 
and  potassium.  Tartaric  acid  (COOH  •  CHOH  •  CHOH  •  COOH)  is 
closely  allied  to  malic  acid.     It  is   found   abundantly  in  the  juice  of 

f  CH2  •  COOH  ] 

grapes  as  potassium-hydrogen  tartrate.  Citric  acid    OH  •  C  •  COOH 

1  CH2  •  COOH  I 

occurs  in  the  juice  of  many  plants,  being  especially  abundant  in  the 
fruits  of  the  citrus  family  (lemon,  lime,  orange,  etc.). 

Tannins.  —  The  tannins  are  numerous  and  widely  distributed,  occur- 
ring especially  in  bark,  wood,  leaves,  fruits,  and  galls.  They  are  bitter 
and  astringent  substances,  which  form  insoluble  compounds  with  pro- 
teins and  gelatin,  and  so  are  used  for  converting  hides  into  leather. 
Tea  leaves  contain  14-16  per  cent  or  more  (dry  weight),  various  barks 
up  to  40  per  cent,  and  galls  up  to  60  per  cent.  Some  substances  included 
in  the  loose  term  tannins  are  glucosides,  and  such  as  can  be  made  to 
yield  glucose  by  digestion  may  be  considered  as  plastic  substances 
rather  than  wastes. 


DESTRUCTIVE    METABOLISM  415 

Alkaloids. — The  alkaloids  arc  numerous,  and  very  Important  medi- 
cinally, as  they  are  dangerous  poisons  <>r  useful  local  stimulants,  ac<  ording 
to  circumstances.  A  few,  such  as  caffein  from  tea  and  coBee,lheobromin 
from  the  seeds  of  cacao  ("cocoa  "),and  the  deadly  muscarin  from  the 
poisonous  mushroom  (Amanita  muscaria),  are  not  related  to  the  alka- 
loids proper,  which  are  for  the  most  part  derivatives  of  pyridin  and  1  bin- 
olin.  The  true  alkaloids  are  found  in  fungi  and  various  seed  plants,  but 
are*  most  common  in  certain  families  of  dicotyls.  For  example,  in  the 
Papaveraceae,  the  oriental  poppy  alone  yields  more  than  twenty  alka- 
loids, of  which  morphin,  narcotin,  and  codein  are  best  known  ;  in  the 
Solanaceae,  tobacco  contains  nicotin  and  others,  and  most  of  the  other 
genera  yield  atropin  and  a  number  allied  to  it;  a  great  number  of  the 
Apocynaceae  have  alkaloids  in  their  latex,  at  least  twenty  different  ones 
being  known;  in  the  Rubiaceae,  the  cinchonas  and  their  allies  produce 
more  than  thirty  alkaloids,  of  which  quinin  and  rim  honin  are  widely 
known;  in  the  Loganiaceae,  seeds  of  Strychnos nux-vomica yield  strych- 
nin and  brurin,  while  another  species  yields  several  "  curare"  alkaloid-,; 
and  in  the  Erythroxylaceae,  coca  yields  among  others  rorain,  at  once 
highly  useful  as  a  local  anesthetic  and  utterly  destructive  to  body  and 
mind  when  used  habitually. 

Coloring  matters  of  flowers,  fruits,  barks,  seeds,  etc.,  are  too  numerous  and 
varied  to  be  discussed  here. 

Ash.  —  Mineral  salts  are  present,  sometimes  amorphous,  incrusting 
or  incorporated  in  the  cell  walls,  as  is  the  case  with  silica;  sometimes 
crystallized,  as  is  the  case  with  calcium  oxalate.  The  ash  of  plants 
consists  of  the  total  mineral  matter  left  as  oxids  when  completely  burned. 
Analysis  shows  that  the  amount  and  content  of  the  ash  varies  much  in 
the  same  plant  in  different  situations,  thus  indicating  that  in  part  (and 
doubtless  in  large  part)  these  materials  are  determined  not  by  the 
"  needs"  of  the  plant  but  by  the  solutions  which  have  opportunity  to 
wander  into  it.  Cultures  under  special  conditions  have  shown  that 
plants  may  be  deprived  of  many  of  the  chemical  elements  ordinarily 
found,  and  no  evil  effects  follow;  but  the  absence  of  others  has  obvious 
ill  effects.  Thus  silica  is  an  abundant  material  in  the  cell  walls  of  the 
epidermis  of  most  cereals;  yet  corn  has  been  cultivated  through  four 
generations  with  practically  no  silica. 

Necessary  elements.  A  list  of  the  elements  that  have  been  found  in 
the  ash  of  one  plant  or  another  would  be  almost  a  li>t  of  the  commoner 


4i6  PHYSIOLOGY 

elements  themselves,  over  thirty  out  of  the  present  total  of  seventy- 
eight  having  been  recorded.  Yet  of  this  large  number  only  a  few  seem 
to  be  indispensable.  These  are  usually  reckoned  as  calcium,  potassium, 
magnesium,  and  iron;  while  chlorin  and  sodium  are  present  in  all  and 
may  be  necessary.  Many  attempts  have  been  made  to  determine  the 
precise  role  of  each  of  these  indispensable  elements,  with  rather  conflict- 
ing results.  It  does  not  seem  possible  by  cultures  which  omit  a  par- 
ticular element  to  reach  reliable  conclusions;  nor  is  it  at  all  likely  that 
the  role  of  any  particular  element  is  simple,  and  the  withdrawal  of  one 
may  permit  others  to  act  in  a  wholly  different  way.  Thus  if  plants  be 
grown  in  solutions  of  calcium  chlorid  or  of  magnesium  chlorid  of  a  cer- 
tain concentration,  they  will  die;  but  if  the  two  be  mixed  in  the  same 
concentration,  the  plants  will  grow  well.  Singly  both  are  injurious, 
together  they  are  not,  though  no  reaction  occurs  between  them. 

When  therefore  it  is  said  that  a  definite  amount  of  each  "  indispen- 
sable "  element  is  needed  by  a  plant,  and  that  the  minimum  determines 
the  crop  ("  law  of  the  minimum  ");  that  on  potassium  depends  the  for- 
mation of  new  organs  at  the  growing  point;  that  calcium  is  required 
for  the  transfer  of  starch,  and  so  on,  all  such  statements  must  be  con- 
sidered as  extremely  doubtful  and  liable  to  complete  reversal  when  a 
deeper  insight  is  gained  into  the  processes  concerned. 


CHAPTER  V.  — GROWTH    AND    MOVEMENT 

i.    GROWTH 

Ideas  involved.  —  Nothing  about  plants  as  a  whole  is  more  readily 
seen  than  that  they  grow,  and  in  due  course  unfold  new  organs.  How- 
ever small  and  simple,  however  large  and  complex,  growth  is  almost 
always  obvious,  and  sometimes  it  becomes  striking  because  of  its  ra- 
pidity or  its  long  duration.  Two  ideas  arc  involved  in  the  term  growth 
as  ordinarily  used,  (</)  an  increase  in  size  and  (l>)  the  formation  of  new 
organs.  The  latter  is  sometimes  distinguished  under  the  term  develop- 
ment, and  if  one  speaks  of  growth  and  development,  the  term  growth  must 
be  limited  to  the  enlargement  of  already  formed  cells.  But  the  terms 
are  nearly  synonymous;  though  growth  may  be  restricted  for  a  time  to 
cells  already  formed,  it  normally  leads  to  the  formation  of  new  organs; 
and  though  development  is  possible  without  enlargement,  it  is  usually 
accompanied  by  an  increase  in  size.  The  production  of  new  organic 
material  is  not  essential;  when  the  corn  seedling,  raised  in  the  dark, 
grows  into  a  plant  many  times  larger,  the  stored  organic  material  has 
been  merely  rearranged,  with  the  addition  of  water,  and  when  the  surplus 
food  has  been  fully  used  for  growth,  there  is  actually  a  smaller  total  of 
dry  matter  than  when  growth  began.  Additional  organic  matter  can 
be  produced  only  when  the  conditions  for  photosynthesis  are  fulfilled. 

Few  plants  have  so  definite  a  cycle  of  development  as  most  animals. 
In  some  cases  leaves  produced  in  the  juvenile  period  differ  from  those  i  f 
later  stages.1  Again,  leaves  developed  at  certain  periods  are  so  different 
in  form  and  texture  as  to  be  really  different  organs,  as  in  the  case  of  bud 
si  ales,  floral  parts,  etc.  But  these  periods  of  flowering  or  seed  formation 
or  other  reproductive  process  are  determined  largely  by  external  con- 
ditions, and  little  or  not  at  all  by  the  fact  that  the  plant  has  reai  hed  a 
certain  stage  of  maturity,  though  of  course  the  formation  of  the  special 
organs,  as  of  all  others,  is  conditioned  by  the  supply  of   constructive 

'These  juvenile  forms,  however,  may  appear  later  under  suitable  conditions.  Sec 
Part  III.  p.  596. 

417 


4i8 


PHYSIOLOGY 


material.  Plants,  therefore,  do  not  in  general  have  a  definite  stage  of 
maturity,  and  a  corresponding  form.  They  do  have,  however,  periods 
characterized  by  growth,  including  the  formation  of  new  organs  and  their 
development.  These  periods  occur  once,  being  limited  to  a  single  season 
or  less,  as  in  the  case  of  annuals;  or  twice,  as  in  biennials;  or  they  are 
repeated,  season  after  season,  as  in  perennials.  This  periodicity  is  less 
marked  in  equable  tropical  climates,  but  is  rarely,  if  ever,  entirely  absent. 
Phases.  —  If  the  history  of  any  limited  portion  of  a  plant  be  followed 
(and  the  more  limited  the  better,  even  to  a  single  cell),  it  can  be  observed 
to  pass  through  a  development  in  which  may  be  recognized  three  phases. 
The  first  phase  may  be  called  the  formative  phase;  the  second,  the  phase 
of  enlargement;  and  the  third  the  phase  of  maturation.  These  phases 
are  characterized  clearly  enough  by  certain  peculiarities  of  structure 
and  behavior,  but  they  are  not  sharply  delimited.  On  the  contrary, 
the  first  passes  by  imperceptible  gradations  into  the  second,  and  the 
second  into  the  third;  then  growth  finally  ceases,  unless  some  unusual 
stimulus  brings  the  cells  again  into  an  active  state. 

Formative  phase. — The  formative  phase  is  the  earliest.    Every  plant 
begins  its  existence  as  a  single  cell,  and  even  when  this  one  has  increased 

to  many,  they  usually  remain 
practically  alike.  The  embryo  in 
seed  plants,  at  the  time  when  it 
resumes  its  interrupted  growth, 
usually  consists  of  cells  all  in 
the  formative  stage.  They  are 
characterized  by  a  relatively  large 
nucleus,  abundant  cytoplasm  with 
only  minute  vacuoles,  and  thin 
walls.  In  this  phase  the  frequent 
division  of  the  cells  is  a  feature, 
and  in  consequence  of  the  more 
rapid  production  of  new  cells  by  division  at  certain  points,  the  primordia 
of  new  organs  appear  (fig.  666).  Some  of  the  simpler  plants  never  get 
beyond  this  phase,  except  as  to  their  reproductive  organs.  Even  in  the 
larger  plants,  some  of  the  cells  permanently  retain  these  characters,  and 
so  constitute  formative  centers  or  growing  points;  but  far  the  greater 
number  pass  gradually  into  the  second  phase  and  the  third,  assuming 
quite  a  different  aspect  and  behavior.  In  particular,  the  power  of 
division  is  given  up. 


Fig.  666.  —  Growin 
•After  De  Bary. 


point  of  Hippuris. 


GROWTH   AND    MOVEMEN1  419 

Primary  meristem.  —  The  formative  regions  in  thallophytes  an-  often 
rather  indefinite,  with  a  tendency  in  the  higher  forms  to  Derestricted 
to  the  apex  of  the  body.  In  the  bryophytes  they  arc  found  only  at  the 
apex,  while  in  the  vascular  plants  they  persist  commonly  at  both  apex 

and  base,  i.e.  at  the  tip  of  each  axis  and  of  each  root.  Here  the  active 
division  of  the  formative  cells  and  the  differentiation  of  their  progeny 
adds  to  the  length  of  the  body  at  one  or  both  ends.  There  may  be  a 
single  cell  acting  as  the  source  of  all,  as  in  ferns,  or  a  group  of  initial-, 
as  in  seed  plants  (fig.  666).  The  repeated  division  of  these  initial-,  and 
their  progeny  being  the  important  feature,  the  formative  tissue  is  des- 
ignated as  meristem,  and  because  this  meristem  persists  from  the  earliest 
stage  in  the  life  history,  it  is  the  primary  meristem. 

Secondary  meristem.  —  Regularly  in  certain  regions  and  accidentally 
in  others,  tissues  that  have  passed  beyond  the  formative  phase  regain  the 
power  of  division  and  exercise  it  for  a  longer  or  shorter  time.  Thus,  in 
all  plants  whose  xylem  and  phloem  bundles  show  secondary  thickening, 
a  layer  of  cells  between  the  two  becomes  a  secondary  meristem  (cambium), 
and  these  initials  may  produce  new  cells  on  either  face  or  both,  which  are 
gradually  transformed  into  elements  like  their  neighbors,  while  the  in- 
itials continue  to  divide  through  the  season,  or  function  year  after  year. 
Again,  a  certain  zone  of  the  cortex  or  even  the  epidermis  itself  may 
resume  active  division,  becoming  a  secondary  meristem  called  the 
phellogen,  whose  offspring,  the  suberized  periderm,  constitutes  a  layer 
of  cork  protecting  the  surface  (see  fig.  539).  Wounds,  the  presence  of  a 
parasite,  or  other  stimuli  may  call  again  into  active  division  almost  any 
live  cells,  and  the  resulting  tissues  will  cover  the  wound  with  a  callus,  or 
produce  the  deformity  characteristic  of  the  particular  injury  or  parasite. 

Origin  of  branches.  —  In  the  primary  meristem  of  the  stem  the  primor- 
dia.  of  new  organs  are  produced  at  the  surface,  the  first  indication  of  a 
new  lateral  branch,  whether  a  shoot  or  a  leaf,  being  a  slight  elevation 
of  the  surface,  due  to  more  rapid  growth  of  cells  at  that  point.  This 
mode  of  origin  is  known  as  exogenous  (fig.  666)  and  is  characteristic  of 
branches  of  the  shoot  axis.  In  the  root,  on  the  contrary,  the  lir>t  ap- 
pearance of  a  lateral  branch  is  not  at  the  surface,  nor  in  the  primary 
meristem,  but  at  the  limit  of  the  stele  or  central  cylinder  (within  the  cor- 
tex), and  among  cells  which  have  given  over  for  a  time  ai  live  division  and 
growth  (fig.  667).  The  new  branch  must  break  through  the  cortex,  since 
it  is  endogenous  in  origin;  and  this  is  characteristic  of  the  root  axis. 
Adventitious  growing  points,  giving  rise  to  new  shoot-,  may  appear  in 


420 


PHYSIOLOGY 


this  endogenous  fashion  upon  roots,  and  likewise  on  old  shoots  or  leaves. 
They  commonly  owe  their  origin  to  some  external  stimulus  (see  p.  428). 
Many  of  the  growing  points  that  are  formed  regularly  (exogenously) 
on  the  shoot  do  not  develop,  for  one  reason  or  another.  They  may  then 
be  overgrown  completely  in  woody  plants,  and  so  lie  dormant  for  years, 
to  be  called  into  activity  when  some  accident  has 
checked  the  growth  of  others,  formerly  more  favor- 
ably situated.  Not  every  shoot,  then,  that  appears 
to  come  from  the  interior  is  really  endogenous  in 
origin. 

Phase  of  enlargement.  —  As  cells  newly  formed 
in  the  meristem  grow  older,  they  enter  gradually 
upon  the  second  phase  of  development.  This  is 
characterized  by  enlargement,  oftentimes  so  great 
and  so  rapid  as  to  be  very  remarkable.  In  this 
period  the  volume  of  the  cell  not  infrequently  in- 
creases a  thousandfold  or  more,  though  ordinarily 
much  less.  Of  course  this  involves  rapid  growth 
of  the  cell  wall  in  area,  and  if  the  cytoplasm  were 
relatively  as  abundant  as  in  the  earliest  stage,  it 
would  require  the  formation  of  a  large  mass  of 
costly  material.  But  while  the  cytoplasm  does 
actually  increase  considerably,  much  the  greater 
part  of  the  cell  is  occupied  by  the  water  which  en- 
ters it.  Hence  an  indispensable  condition  for  growth  is  an  adequate  supply 
of  water;  and  the  dwarfing  which  results  from  a  deficiency  of  water  is 
partly  a  direct  consequence  of  the  non-distension  of  the  cells  in  this  stage. 
The  water  enters  the  protoplasm,  doubtless  as  a  result  of  the  formation  of 
substances  having  a  high  osmotic  pressure.  It  enlarges  the  minute  vacu- 
oles everywhere  through  the  cytoplasm,  until  some  become  so  distended 
as  to  merge,  forming  fewer  but  larger  ones.  This  process  continues  until 
in  the  center  a  few  large  vacuoles,  or  often  only  one,  occupy  the  greater 
part  of  the  space,  while  the  major  portion  of  the  cytoplasm  lies  next 
the  cell  wall  as  a  relatively  thin  layer,  containing  the  nucleus,  plastids, 
and  other  inclusions  (see  diagram,  fig.  619).  It  will  be  apparent  that 
since  this  many-fold  enlargement  is  attained  so  largely  at  the  expense  of 
water,  plant  growth  is  relatively  economical. 

Unequal  enlargement.  —  The  young  cell  has  its  three  dimensions 
nearly  equal.     Enlargement  takes  place  in  all  dimensions,  but  to  different 


Fig.  667.  —  Endoge- 
nous origin  of  a  lateral 
root  (r)  of  ice  plant  [Me- 
sembryanthemun  crystal- 
linum) :  c,  primary  cor- 
tex, and  e,  endodermis, 
ruptured  by  young  root; 
p,  pericycle,  from  which 
it  arises;  X,  primary  xy- 
lem  element. — After  Van 
Tieghem  and  Douliot. 


GROWTH    AND    MOVEMENT  421 

degrees,  according  to  circumstances.  Thus,  cells  which  arc  part  of  an 
elongated  organ  like  a  stem,  arc  likely  to  grow  mm  h  more  in  the  longi- 
tudinal diameter  than  the  transverse.  The  real  reason  for  these  ine- 
qualities of  growth  is  obscure.  To  say  that  they  are  due  to  "  inherent 
causes"  or  are  determined  by  "heredity"  in  no  wise  enlightens  the 
inquirer.  In  a  few  cases  they  are  referable  to  definite  agencies.  Thus, 
the  <  ells  near  the  upper  surface  of  a  leaf  are  influent  ed,  mainly  by  light, 
to  grow  longer  in  the  axis  at  right  angles  to  the  surface  than  in  the  other 
two.1  The  sum  total  of  growth  in  the  individual  cells  determines  in  large 
measure  the  final  form  of  the  organ  in  which  they  lie.  In  most  cases  the 
causes  which  determine  the  general  course  of  growth  can  be  analyzed  at 
present  as  little  as  those  which  determine  the  form  of  the  single  cell; 
but  the  effect  of  external  agents  is  often  detected,  and  in  many  <  ases 
it  is  dominant  (see  section  3,  p.  435). 

Grand  period.  —  Enlargement  proceeds  at  an  unequal  pace,  even 
though  the  external  conditions  which  affect  the  rate  are  kept  uniform. 
In  the  earlier  portion  of  the  period  it  is  slow,  then  it  becomes  more  and 
more  rapid  until  it  attains  a  maximum,  when  it  quickly  falls  off  and 
gradually  comes  to  an  end.  If  the  progress  is  graphically  represented 
by  plotting  the  increment  from  day  to  day,  a  curve  is  obtained  of  which 
fig.  668  is  an  example.  This  is  the  history,  indeed,  of  the  growth  in 
length  of  a  short  portion  of  a  stem,  which  is  made  up  of  a  multitude  of 
cells  in  the  phase  of  enlargement.  In  a  similar  way  the  growth  in  volume 
of  a  fruit,  such  as  an  apple  or  a  pumpkin,  might  be  described.  The 
total  period  of  enlargement  is  named  the  grand  period  of  growth,  to  dis- 
tinguish it  from  periodic  variations  in  the  rate  within  the  grand  period, 
some  of  which  are  due  to  periodically  acting  external  agents,  such  as 
light  and  heat  (daily  period,  see  p.  436),  and  others  to  causes  unknown 
and  hence  called  "  spontaneous  "  variations. 

The  same  features  of  the  course  of  growth  may  lie  seen  when  the  in.  remenl  <>f 
successive  small  portions  of  an  axis  is  rec  orded.  Thus  if  a  root  is  marked  into  milli- 
meter spa  is,  or  a  stem  into  longer  spaces  ami  the  in<  remenl  of  ea  h  is  ro  orded  for 
a  number  <>f  hours,  it  will  appear  that  certain  spaces  are  growing  more  rapidly  than 
others,  respe<  lively  more  or  less  distant  from  tin-  tip,  ;'./•.  older  or  younger. 

The  increment  in  twenty-four  hours  of  each  of  ten  1  mm.  spa  esof  a  root 
is  here  shown: 


I 

II 

III 

IV 

V 

VI 

VII 

VIII 

IX 

X 

■s 

5-8 

82 

3-5 

1.6 

1.3 

0.5 

0..? 

0.2 

O.I 

1  Transpiration  may  Ik-  another  f.utor;  the  precise  relation  of  tin-  two  is  uncertain.   See 
fart  III,  p.  536. 


422 


PHYSIOLOGY 


Similarly  the  increase  in  forty  hours  of  twelve  3.5  mm.  spaces  of  a  stem  of 
Phaseolus: 


1  II        III       IV        V         VI      VII     VIII        IX      X         XI      XII 

2  2.5        4.5        6.5       5.5        3.0        1.8        1.0         1.0      0.5        0.5       0.5 

Inspection  of  these  records  shows  that  the  two  younger  millimeters  of  the  root 
and  the  seven  older  are  growing  less  rapidly  than  the  third  ;  in  the  stem  the  four- 
teenth to  the  seventeenth  millimeters  (space  IV)  are  growing  most  rapidly,  and 
beyond  this  the  older  a  division  is  the  more  slowly  it  grows. 

Growing  regions.  —  Comparison  of  the  total  length  of  root  and  stem 
still    growing   appreciably  shows  a  striking  difference.     About   1    cm. 


75° 
70° 

, 

S 

\ 

y'' 

\ 

'"" 

\ 

^ 

--J 

"-> 

— 



70 

60 

1 

/ 

50 

/ 

_J 

/ 

40 

I 

\ 

/ 

\ 

30 

f 

> 

\ 

20 
t 

MM 

DAYS-*  1 


10 


»2       13      14       15 


Fig.  663.  —  Grand  curve  of  growth  (solid  line):  the  first  day  of  the  observation  was 
evidently  after  fairly  rapid  growth  had  begun;  it  attained  a  maximum  on  the  fifth  day, 
with  an  increment  of  72  mm.;  thence  the  rate  falls  off  rapidly,  and  on  the  sixteenth  day 
is  only  18  mm.;  growth  rate  magnified  10  times.  The  temperature  curve  (broken  line) 
for  the  same  days  runs  between  71  and  770  F.  —  From  data  by  Spoehr. 

of  the  root  and  more  than  4  cm.  of  the  stem  is  shown  to  be  growing 
by  the  record  above.  In  general  the  total  elongating  portion  of  a  root 
scarcely  exceeds  this;  but  in  many  stems  10-20  cm.  are  found  elon- 
gating, and  in  twining  plants  40-60  or  even  80  cm.  may  be  growing. 


GROWTH   AND   MOVEMENT  423 

The  growth  of  aerial  stems  is  not  hindered  by  the  medium.  When  they 
grow  underground,  the  apex  is  protet  ted  by  a  duster  of  overarching  s<  ale-. 
Growth  of  such  stems  is  seldom  rapid,  but  when  it  is,  as  in  the  extensive 
running  rootstocks  of  couch  grass,  the  terminal  hud  is  sharp-pointed  and 
smooth,  so  that  it  offers  the  least  resistance  to  being  driven  through  the 
soil;  at  the  same  time  the  firm  scales  protect  the  primary  meristrm 
behind.  In  the  root  it  is  obviously  advantageous  to  have  the  growth  zone 
restricted,  and  to  have  the  zone  of  most  rapid  growth  as  near  the  apex 
as  possible;  for,  so  much  as  any  part  behind  it  elongates,  so  far  is  the 
tip  actually  driven  through  the  soil.  The  sloughing  and  slimy  surface 
of  the  root  cap  lubricates  the  advancing  apex,  thus  facilitating  its  pas- 
sage. For  good  growth  of  roots  (which  makes  for  good  growth  above 
also),  it  is  desirable  that  the  soil  have  an  optimum  content  of  water, 
since  it  has  been  shown  that  its  resistance  to  penetration  is  then  at  a 
minimum.  Drought,  indeed,  hinders  root  growth  doubly;  it  not  only 
retards  enlargement  directly  by  lack  of  water,  but  also,  by  compacting 
most  soils,  mechanically  opposes  the  extension  of  the  root  system,  and 
so  intensifies  the  difficulty  of  procuring  the  necessary  water. 

Nutations.  —  The  rate  of  elongation  is  not  only  different  in  different 
sections  along  the  axis;  it  is  also  unequal  in  different  segments  around 
the  axis.  This  is  especially  marked  in  bilateral  organs,  such  as  leaves, 
and  varies  from  one  face  to  another  at  different  periods  of  development. 
Thus,  most  leaves  when  young  grow  more  rapidly  on  the  back  (later  the 
under  surface),  so  that  they  are  appressed  to  the  stem;  or  they  arch  over 
its  apex  when  they  outgrow  it,  as  they  commonly  do,  forming  a  "  bud  " 
there.  Later,  growth  becomes  more  rapid  on  the  inner  face  (at  matur- 
ity the  upper  surface)  and  the  bud  opens.  Local  differences  in  rate  lead 
to  the  folding  and  rolling  so  characteristic  of  young  leaves  in  the  bud. 
In  radially  symmetrical  organs,  such  as  stems,  inequality  of  growth 
on  different  radii  leads  to  bending,  so  that  the  tip  is  not  erect  but  more 
or  less  declined.  As  the  most  rapid  growth  shifts  to  different  segments 
around  the  axis,  the  tip  nods  successively  to  all  points  of  the  compass, 
and  so  describes  a  very  irregular  ellipse  ,,r  ( in  le,  or,  considering  also  its 
upward  growth,  a  very  irregular  ascending  spiral.  Plotting  successive 
observations  on  a  plane  shows  tracings  like  fig.  669.  The  nodding  of 
leaves  or  stems  or  roots  on  account  of  unequal  growth  is  1  ailed  nutation. 
The  inequalities  in  the  rate  of  growth  may  be  due  to  unknown  causes, 
assumed  to  be  internal,  when  the  corresponding  nutation  is  (ailed  spon- 
taneous or  autonomic;   or  they  may  be  due  to  external  causes  (stimuli), 


424 


PHYSIOLOGY 


when  the  nutations  are  said  to  be  induced.     The  latter  will  be  particu- 
larly discussed  later  (see  section  4,  p.  442,  and  section  7,  p.  458). 

Rapidity.  —  The  absolute  rate  of  growth  in  the  period  of  enlargement 
is,  of  course,  extremely  different  in  different  plants  and  under  different 
conditions.  A  few  cases  may  give  an  idea  of  the  upper  limits.  The 
filaments  of  wheat  stamens  at  the  time  of  blooming  grow  for  a  brief  time 
at  the  rate  of  1.8  mm.  per  minute,  which  is  about  the  rate  at  which  the 
minute  hand  of  a  man's  watch  travels.     If  such  a  rate  continued  for 

24  hours,  they 
would  become 
2.5  m.  long. 
The  leaf  sheath 
of  the  banana 
grows  at  the  rate 
of  1.1  mm.  and 
that  of  bamboo 
0.6  mm. per  min- 
ute. When  the 
century-plant 
blooms  (as  it 
does  in  10-25 
years),  a  shaft 
about  15  mm. 
in  diameter  rises 
to  a  height  of 
6-8  m.  at  the 
rate  of  about 
15  cm.  per  day. 

Phase  of  maturation.  —  The  phase  of  maturation  is  the  final  phase  of 
growth.  This  phase  is  entered  upon  only  when  enlargement  has  prac- 
tically ceased;  therefore  its  progress  is  not  measurable,  though  it  is  quite 
as  important  as  the  preceding.  During  this  phase  the  cells  attain  their 
mature  form  and  character.  In  all  cases  the  thickening  of  the  cell  wall 
is  obvious,  though  often  slight;  but  sometimes  it  proceeds  to  such  an 
extreme  as  to  be  the  most  notable  change.  The  thickening  is  never  uni- 
form, and  sometimes  thin  and  thicker  spots  in  patterns  produce  an  effect 
of  sculpturing  that  is  characteristic,  as  in  the  tracheae  and  tracheids 
(figs.  640,  641).  Conversely  the  resorption  of  certain  parts  of  the  wall 
may  occur,  as  the  end  partitions  of  sieve  tubes  and  of  the  components  of 


Fig.  669.  — Nutations  of  a  young  sunflower  plant:  1  position  at 
9  A.M.,  2  9:15,  3  9:30,  4  9:45.  5  10:00,  6  10:15,  7  10:30,  8  11  :oo, 
9  11:30,70  12  M.,  11  1:00  p.m.,  12  2:00;  from  point  12  the  plant 
made  a  deep  nod  to  the  west  till  4  P.M.,  then  again  eastward  till 
5:00,  again  westward  till  6:00,  and  finally  to  original  meridian  at 
9:00  p.m.  —  From  data  by  Land. 


GROWTH   AND   MOVEMENT  425 

tracheae  and  the  thin  portions  of  the  wall  in  the  scalariform  tracheids 
of  ferns.  In  case  great  thickening  occurs,  the  death  of  the  protoplast 
is  likely  to  follow,  and  tin's  is  regularly  the  case  in  tracheary  tissue. 
When  that  occurs,  further  modification  of  the  wall  is  possible  only  by 
the  agency  of  adjacent  live  cells,  by  chemical  reaction  in  the  wall  sub- 
stances, or  by  mere  impregnation  with  solutes  which  may  be  precipitated 
or  absorbed.  So  proceed  such  changes  as  the  coloring  and  other  altera- 
tions which  mark  the  heart  wood  of  trees. 

Tension  of  tissues.  —  When  growth  is  finally  at  an  end  in  any  region, 
it  is  found  that  the  various  tissues  have  not  grown  equally.  Hence  there 
exist  strains  or  tensions;  one  region  is  compressed,  another  is  stretched. 
These  inequalities  tend  to  adjust  themselves  if  tin-  regions  are  parted 
anitu  tally,  as  when  the  pith,  the  bark,  and  the  wood  are  separated  from 
one  another.  Similarly,  tensions  due  to  unequal  turgor  exist  (see  p.  310). 
All  these  strains  acting  in  different  directions  within  the  structure  tend 
to  increase  its  rigidity,  just  as  do  like  strains  in  a  latticed  girder  or  a 
bridge  truss. 

Conditions.  —  The  conditions  for  growth  are  first  of  all  an  adequate 
supply  of  water,  for  unless  turgor  of  a  meristem  region  is  maintained, 
division  of  the  cells  is  impossible,  and  unless  an  adequate  amount  of 
water  be  present,  enlargement  of  formed  cells  is  limited.  Secondly, 
there  must  be  a  sufficient  supply  of  constructive  materials  ;  for  though 
water  plays  an  extraordinary  part  in  enlargement,  there  is  needed  much 
food  for  making  new  cytoplasm  as  new  cells  arise  by  division  and  en- 
large. Nuclear  material,  cell-wall  stuff,  and  much  besides  must  be 
steadily  constructed  by  the  protoplasts,  and  the  growing  region  is  there- 
fore the  seat  of  intense  chemical  activity.  Thirdly,  oxygen  is  necessary, 
probably  to  permit  the  metabolism  in  general,  and  especially  the  res- 
piratory changes,  to  proceed  properly.  For  though  growth  has  been 
observed  in  the  absence  of  oxygen,  it  is  quite  limited,  and,  having  been 
detected  only  by  measurement,  was  probably  due  solely  t>>  the  disten- 
tion by  water.  Cell  division  also  is  checked  by  lack  of  Oj.  Lastly, 
growth,  like  all  other  phenomena,  goes  on  only  within  certain  limits 
of  temperature,  other  conditions  being  suitable.  The  optimum  (dif- 
ferent for  different  plants  and  for  the  same  plant  under  different  con- 
ditions) usually  lies  between  250  and  320  C,  and  the  extremes  are  near 
o°  and  420  C.  Any  one  of  the  conditions  named  may  likewise  vary 
within  rather  wide  limits,  and  any  one  being  unfavorable  may  retard 
or  stop  growth.     Yet  when  all  the  condition^  are  favorable,  periodic 


426  PHYSIOLOGY 

variations  still  mark  the  rate  of  growth,  indicating  clearly  that  there 
are  unknown  factors  that  operate  with  or  against  the  known  factors  to 
affect  it.  The  existence  of  such  unknown  influences  is  further  shown 
by  the  fact  that  growth  ceases,  sooner  or  later,  in  individual  cells,  and 
often  in  the  whole  plant,  in  spite  of  all  efforts  to  supply  appropriate 
conditions. 

External  agents.  —  A  study  of  growth  shows  that  external  agents 
produce  obvious  effects.  They  do,  indeed,  affect  every  function,  and 
much  investigation  is  still  necessary  before  the  full  extent  of  their  influ- 
ence is  known.  But  growth  is  at  once  so  fundamental  and  so  easy  to 
observe,  that  it  affords  the  best  means  for  showing  how  extraordinary  a 
part  external  agents  play  in  determining  the  form  and  behavior  of  plants. 
To  this  phase  of  plant  life  attention  must  now  be  directed. 

2.    IRRITABILITY 

External  agents.  —  It  is  a  matter  of  common  observation  that  the  size 
and  form  of  plants  is  affected  by  the  conditions  under  which  the)  are 
grown.  The  luxuriance  of  weeds  in  a  neglected  garden,  in  contrast  with 
their  stunted  forms  on  a  dry  roadside  ;  the  rich  green  corn  of  a  high 
prairie,  in  contrast  with  the  yellowish  and  starved  plants  on  a  wet  clay 
field  ;  the  thrifty  trees  of  a  park,  in  contrast  with  the  struggling  and 
dying  ones  along  a  paved  street,  can  hardly  fail  of  notice  by  the  most 
unobservant.  These  differences  show  clearly  that  the  complex  of  con- 
ditions external  to  the  plant  profoundly  affects  its  internal  processes. 
As  all  functions  center  in  the  living  stuff,  protoplasm,  the  conclusion  is 
that  protoplasm  is  sensitive  to  the  various  agents  that  act  upon  it  (or 
irritable);  that  is,  that  it  reacts  or  responds  to  these  by  altering  its  be- 
havior in  some  way.  In  that  event  the  agent  producing  the  reaction  is 
a  stimulus.  These  three  topics,  stimulus,  response,  and  sensitiveness  or 
excitability,  require  consideration. 

Variety  of  stimuli.  —  The  forces  that  act  upon  any  plant  are  many, 
and  varied  in  direction  and  intensity;  and  their  combinations  are  almost 
infinite.  Consider  a  tree,  growing  in  a  Chicago  park.  Every  day  the 
light  which  falls  on  it  varies  both  in  direction  and  in  intensity  from 
hour  to  hour,  and  is  almost  lacking  at  night;  furthermore  it  varies  from 
day  to  day  and  season  to  season.  The  temperature  is  hardly  the  same 
from  one  hour  to  another,  and  in  this  climate  occasionally  changes 
io°  C.  within  twice  as  many  minutes,  while  the  seasonal  changes  range 


GROWTH   AND   MOVEMENT  427 

over  some  700  C.  The  humidity  of  the  air  shows  like  hourly,  daily,  and 
seasonal  fluctuations,  and  the  tree  may  he  thrashed  by  a  pan  hing  wind 
or  wrapped  in  a  dripping  fog.  A  gentle  shower,  torrential  rain,  or  hail 
may  fall  upon  it  within  the  hour;  and  with  a  change  of  season  it  may  be 
weighed  down  by  sleet  and  snow.  The  underground  parts  suffer  less 
extreme  variations  of  temperature  than  the  top.  The  water  ion  tent 
of  the  soil  swings  from  the  drought  of  summer  to  the  saturation  of  late 
winter  and  spring,  and  the  solutes  vary  more  or  less  in  concentration 
with  the  rains  and  evaporation.  Combine  all  these  in  as  many  ways  as 
possible,  and  some  idea  is  obtained  of  the  variations  in  external  con- 
ditions which  may  affect  the  plant. 

Adjustment.'  —  To  many  of  these  a  plant  must  be  able  to  adjust  itself 
on  pain  of  death,  and  suitable  response  to  others  is  advantageous.  The 
plant  is  indeed  a  self-adjusting '  mechanism,  whose  reactions  are  often- 
times more  delicate  than  those  of  our  own  bodies,  with  all  their  special 
senses  and  complicated  sense  organs.  Thus,  many  a  tendril  is  sensitive 
to  a  mechanical  stimulus  which  we  cannot  perceive,  even  by  the  tip  of 
the  tongue,  the  portion  of  the  body  most  sensitive  to  contact ;  and  some 
plants  distinguish  differences  of  illumination  which  are  inappreciable  to 
the  eye.  On  the  whole,  it  is  perhaps  fair  to  say  that  plants  are  more 
responsive  than  animals.  The  plant  has  mostly  to  take  what  comes  and 
make  the  best  of  it;  the  animal  often  takes  shelter  from  unfavorable 
conditions  or  migrates  to  a  gentler  climate. 

Intricate  relations.  —  It  is  extremely  difficult  to  disentangle  the  com- 
plex of  forces  acting  on  a  plant  and  to  assign  to  each  its  special  influence. 
Out  of  them  all  only  a  few  have  yet  been  isolated.  What  are  known 
as  general  or  formative  stimuli,  namely,  the  totality  of  physical  conditions, 
external  and  internal,  which  determine  the  general  course  of  develop- 
ment and  consequently  the  form  of  the  plant  as  a  whole  or  of  any  par- 
ticular organ,  furnish  especially  intricate  problems,  because  it  is  so  dif- 
ficult to  alter  only  one  condition  experimentally,  or  to  evaluate  the 
influence  of  those  which  cannot  be  controlled.  Experience  is  showing, 
too,  that  so-called  special  stimuli,  i.e.  those  which  act  locally,  such  as 
gravity,  light,  heat,  etc.,  are  interrelated,  and  their  effects  are  unexpei  1 
edly  interwoven.  No  phase  of  plant  life  requires  more  cartful  experi- 
mentation and  more  caution  in  inference  than  the  study  of  stimuli  and 
the  responses  to  them. 

1  This  term  must  be  understood  as  if  it  were  applied  to  a  steam  engine  or  a  dynamo, 
both  of  which  adjust  themselves  automatically  to  their  "  load." 


428  PHYSIOLOGY 

Definition.  —  A  stimulus  is  any  change  in  the  intensity  or  direction  of 
application  of  energy  which  produces  an  appreciable  effect  upon  living 
protoplasts.  Of  course  when  no  appreciable  effect  is  produced,  the 
energy  may  differ  neither  in  amount  nor  form  from  that  which  does 
arouse  a  reaction;  and  effects  may  be  produced  which  are  not  perceived 
because  improper  tests  are  applied.  A  stimulus,  thus,  has  no  absolute 
value;  it  implies  not  a  definite  amount  of  energy  measured  in  physical 
units,  but  merely  enough  applied  suddenly  enough  to  call  forth  a  reaction 
as  revealed  by  some  arbitrary  test.  Therefore,  what  is  a  stimulus  under 
certain  conditions,  is  not  a  stimulus  under  others.1  Nor  need  the  stimu- 
lus arise  or  act  outside  the  plant  as  a  whole.  It  may  originate  in  one  part 
and  act  upon  an  adjacent  part,  even  in  one  protoplast  and  act  upon 
another.  These  stimuli,  in  one  sense  external  and  in  another  internal, 
are  most  difficult  to  study.  They  are  in  part,  and  perhaps  wholly,  the 
occasion  for  the  reactions  that  are  called  autonomic,  or  less  properly 
"  spontaneous." 

Kinds.  —  Stimuli  may  be  classified  for  convenience  as  mechanical, 
chemical,  and  ethereal.  Under  mechanical  stimuli  are  grouped  those 
which  depend  upon  mass  movements,  resulting  in  contact,  impact, 
friction,  pressure,  etc.,  upon  the  plant.  For  lack  of  definite  knowledge 
of  the  nature  of  gravitation,  the  stimulus  of  gravity  may  be  conveniently 
included  here,  since  it  depends  upon  mass  attraction  and  induces  mass 
movements.  Under  chemical  stimuli  are  included  those  whose  action 
depends  on  their  chemical  quality  —  their  composition  and  molecular 
structure  —  rather  than  on  their  mass.  Ethereal  stimuli  comprise 
those  propagated  as  vibrations  in  the  ether  and  distinguished  according  to 
the  length  of  the  waves  as  light,  heat,  and  electricity. 

Modes  of  reaction.  —  The  action  of  a  stimulus  results  in  stimulation 
or  excitation,  and  this  may  or  may  not  lead  to  an  observable  reaction, 
depending  upon  the  state  of  the  protoplasm  and  the  means  used  to  detect 
a  change  in  its  behavior.  Thus,  immediately  upon  excitation  a  change 
in  the  electrical  condition  of  the  protoplast  occurs,  but  this  does  not  mani- 
fest itself  to  our  senses,  unless  the  stimulated  region  and  an  unstimulated 
one  are  put  into  electrical  connection  with  the  poles  of  a  sensitive  gal- 
vanometer (fig.  670).     At  the  same  moment  a  contraction  of  the  proto- 

1  No  sharp  distinction  can  be  drawn  between  the  stimuli  which  are  followed  by  a 
prompt  and  easily  observable  response  and  those  external  agents  whose  very  gradual 
change  has  no  early  apparent  effect,  but  produces  ultimately  some  deviation  from  the 
usual  course  of  development.  In  the  broad  sense  both  are  stimuli,  but  the  term  is 
usually  applied  only  to  the  former,  in  which  sense  it  is  here  defined. 


GROWTH    AND    M<  >VI  MINT 


429 


plasts  occurs,  and  this  may  or  may  not  be  apparent.  It  expres  1 
by  a  change  of  position  in  the  leaf  of  Biophytum  (fig.  070),  or  of  Mimosa 
because  there  is  at  the  base  of  the  leaf  a  cushion  of  cells,  whose  lower 
ones,  on  account  of  the  stimulation,  exude  some  of  the  water  that  kept 
them  tense  mi. re  readily  than  do  the  upper  ones.  Again,  upon  stimu- 
lation there  may  be  a  <  hange  in  the  rate  or  amount  of  some  function  or, 
more  rarely,  a  change  in  the  character  of  a  function.  Thus,  the  proto- 
plasm of  a  gland  may  be  caused  to  secrete  more  or  less  rapidly  than 
before,  or  the  protoplasm  in  a  growing  cell  may  have  its  growth  accel- 
erated or  retarded.     Further,  a  gland  may   have   the  character  of  its 


0  l'  V  Z' 

FlG.  (>~o.  —  Records  of  simultaneous  mechanical  (M)  and  electrical  (E)  response  in 
Biophytum;  the  figures  are  seconds;  dotted  lines  show  the  moment  of  application  of  a 
stimulus,  ami  the  solid  lines  the  deflection  of  the  leaflet  or  of  the  galvanometer  needle. 
-After  Bose. 

secretion  profoundly  altered  by  excitation,  or  a  part  not  growing  may  have 
its  cells  set  again  into  active  division  and  growth. 

Sensitive  plants.  —  The  fact  that  certain  plants,  having  a  special 
mechanism,  respond  to  a  stimulus  quickly  by  a  mechanical  movement 
has  given  them  an  undeserved  reputation  as  "  sensitive  plants  "  par  ex- 
cellence; but  they  are  not  really  more  sensitive  than  others.  Whether 
a  plant  exhibits  movements  or  not  depends  on  whether  it  has  an  ap- 
propriate mechanism  to  permit  the  protoplasmic  contractions  to  propel 
it  through  the  water,  or  the  changed  turgor  to  displace  an  organ,  or  the 
changed  rate  of  growth  to  cause  a  curvature.  Movements,  then,  are 
favorable  for  a  study  of  sensitiveness  merely  because  they  are  obvious 
reactions  that  can  often  be  observed  without  apparatus.  They  do  not 
signify  unusual  sensitiveness,  nor  does  immobility  imply  its  lack.  Every 
plant  responds  appropriately  to  a  sufficient  stimulus,  and  every  plant  is 
therefore  a  sensitive  plant. 

Propagation  of  the  excitation. — The  reaction  specially  observed  is 
nut  usually  the  only  one.     It  may  be  only  one  of  a  st  ties,  and  curvature, 


430  PHYSIOLOGY 

resulting  in  movement,  is  most  likely  to  be  merely  the  end  reaction. 
Thus  if  a  primary  root  of  a  bean  be  set  horizontal,  the  first  reaction 
occurs  instantly  and  in  the  very  tip  of  the  root,  but  it  is  not  visible;  only 
after  a  half  an  hour  or  more,  at  a  distance  of  2-3  mm.  from  the  tip,  does  a 
growth  reaction  set  in  that  starts  to  turn  the  root  tip  downward.  Between 
the  first  reaction  and  the  last  there  must  have  been  a  series  of  changes, 
each  of  which  was  a  reaction  to  a  preceding  stimulus  and  a  stimulus  to 
a  succeeding  reaction.  By  a  rough  analogy  the  process  may  be  com- 
pared to  the  tumbling  of  a  row  of  blocks,  each  falling  by  reason  of  the 
impulse  from  its  predecessor  and  impelling  its  successor  to  fall.  The 
push  that  displaced  the  first  one  is  the  primary  stimulus,  and  if  the  last 
were  properly  connected  mechanically,  it  might,  for  the  end  reaction, 
ring  a  bell  or  fire  a  gun.  Such  a  series  of  reactions  is  often  spoken  of  as 
the  transmission  of  the  stimulus.  More  properly  it  is  the  propagation 
of  the  excitation.     It  is  equally  the  propagation  of  a  reaction. 

None  of  these  phrases  nor  the  above  analogy  should  be  understood  to  require 
that  the  reactions  in  a  series  are  necessarily  alike,  nor  is  the  end  reaction  the  only 
one  to  which  the  term  properly  belongs,  though  it  is  usually  so  applied  unless  the 
contrary  is  indicated. 

Perceptive  region.  —  The  region  where  the  first  reaction  occurs  is  often 
called  the  receptive  or  perceptive  x  region,  particularly  if  a  later  and  ob- 
vious end  reaction  occurs  at  another  place.  Since  in  animals  a  similar 
localization  of  sensitiveness  for  special  stimuli  marks  the  peripheral  por- 
tion of  sense  organs,  these  regions  in  plants,  especially  when  very  cir- 
cumscribed, may  be  looked  upon  as  sensory  organs  of  the  simplest  sort.2 
Regions  of  this  sort,  sensitive  to  gravity  and  light  as  stimuli,  will  be 
described  later  (pp.  463,  477).  In  the  great  majority  of  cases,  however, 
perception  is  not  strictly  localized,  and  the  condition  resembles  rather 
that  in  the  diffuse  senses  of  animals,  like  those  of  touch  and  temperature. 

Transmission.  —  Special  tracts,  the  nerves,  exist  in  almost  all  animals, 
along  which  the  excitation  is  propagated,  but  nothing  at  all  comparable 
has  been  found  in  plants,  though  this  claim  has  been  made  more  than 
once.  The  most  that  can  be  said  is  that  propagation  is  more  rapid 
lengthwise  than  crosswise  of  the  cells  of  a  tissue  and  in  some  tissues  is 
easier  than  in  others.  Presumably  the  propagation  is  from  protoplast 
to  protoplast  by  way  of  the  slender  threads  that  connect  them,  traversing 

1  These  words  are  used  in  a  figurative  sense,  and  the  last  must  not  be  understood  to 
have  its  usual  psychological  implication. 

2  Here  again  it  is  necessary  to  point  out  that  in  no  sense  is  consciousness  implied. 


GROWTH    AND    MOVEMENT 


43] 


the  walls.  It  is  do!  at  all  certain  that  there  are  not  other  more  me- 
chanical means  of  transmitting  the  disturbance  thai  eventuates  in  move- 
ment.1 

Responsive  region.- — Corresponding  to  the  perceptive  region,  the 
place  where  the  final  reaction  occurs  is  called  the  active  or  responsive 

region.  Of  course  it  is  not  more  active  or  responsive  than  the  inter- 
vening regions;  but  attention  is  fixed  on  it  as  the  seat  of  the  selei  ted 
reaction.  Thus,  in  the  root  above  referred  to,  the  perceptive  region  is 
in  the  root  cap,  the  excitation  is  propagated  backwards  through  several 
millimeters  of  meristematie  cells  to  those  in  the  phase  of  enlargement, 
and  the  region  of  most  rapid  growth  is  the  responsive  region,  because 
there  the  growth  rate  is  unequally  affected  on  the  upper  and  under  side, 
and  so  a  curvature  appears  in  that  zone,  which  turns  the  tip  downward 
again. 

Mechanism  of  reaction.  —  Consideration  of  even  one  such  curvature 
shows  that  the  nature  of  the  reaction  is  in  no  way  determined  by  the 
nature  of  the  stimulus,  since  the  same  stimulus  produces  a  number  of 
reactions  differing  entirely  from  the  end  reaction,  curvature.  When 
many  movements  are  studied,  this  feature  appears  most  strikingly,  for 
it  is  seen  that  the  same  stimulus  may  produce  curvatures  in  exactly 
opposite  directions  in  different  parts,  such  as  a  root  and  a  shoot,  while 
different  stimuli  may  call  forth  identical  responses.  Further,  stimuli 
of  the  same  sort  at  different  intensities  may  call  forth  opposite  reactions. 
The  mode  of  action  is  determined  in  fact  by  the  mechanism  concerned. 
Just  as  an  electric  current  may  ring  a  doorbell,  Mart  an  engine,  or  ex- 
plode a  mine,  according  to  the  mechanism  at  the  end  of  the  wire;  so  an 
electric  current  may  shorten  a  stamen,  drop  a  leaf,  or  curve  a  tendril, 
according  to  the  mechanism  set  into  operation  in  the  plant.  Vet  prob- 
ably there  is  some  effect,  fundamentally  similar  in  each  case,  which  works 
out  to  a  different  final  result,  just  as,  in  the  comparison,  the  magnetizing 
of  an  iron  bar  underlies  the  varied  results. 

Tropic,  nastic,  taxic  movements.  —  In  some  cases,  however,  the  stimu- 
lus in  a  measure  controls  the  rea<  tion.  A  stimulus  that  acts  upon  plants 
from  a  definite  dire,  tion,  and  consequently  from  one  side,  may  deter- 
mine by  that  fact  the  plane  of  the  consequent  curvature,  provided  the 
organ  be  physiologically  radial,  i.e.  capable  of   response  in   any  plane. 

1  The  "nerves"  of  leaves  are  so  called  only  b  ■  relative  terms,  "veins" 

and  "rilis,"  imlii  ate.  They  probably  have  nothing  to  '1"  with  transmitting  an  ezi  itation 
in  ordinary  .  ases,  though  some  r«  ent  ob  •  i  ration    alb  ge  the  i  ontrary, 


432 


PHYSIOLOGY 


Such  curvatures  are  called  in  general  tropic  and  the  phenomena  tropisms. 
To  these  terms  is  often  prefixed  a  word  indicating  the  stimulus  which 
calls  forth  the  tropism,  as  geotropism  (ge,  the  earth  =  gravity),  photo- 
tropism  (photos,  light),  etc.  (see  p.  458).  When  a  curvature  evoked  by 
either  a  uniform  or  a  one-sided  stimulus  is  restricted  to  a  single  plane  by 
the  bifacial  structure  of  the  organ,  the  curvatures  are  called  nastic,  and 
the  phenomena  nasties.  This  term  is  also  applied  to  like  curvatures 
due  to  unknown  ("  internal  "  or  "  inherent  ")  causes.  Thus  we  have 
epinasty  and  hyponasty,  photonasty,  photepinasty,  etc.  (see  further, 
p.  442).  In  the  organisms  capable  of  locomotion,  a  one-sided  stimulus 
may  determine  the  direction  of  creeping  or  swimming.  These  phenom- 
ena are  taxic,  collectively  taxies,  and  individually  chemotaxy,  phototaxy, 
geotaxy,  etc.,  according  to  the  stimulus  (see  p.  446). 

Energy  relations.  —  Not  only  is  the  mode  of  reaction  independent  of 
the  kind  of  stimulus,  but  its  energy  is  disproportionate  to  the  amount  of 
energy  expended  in  excitation.  The  stimulus,  therefore,  cannot  be  the 
sole  cause  of  the  reaction,  though  the  two  stand  related  to  each  other 
apparently  as  cause  and  effect.  On  the  unexpected  pricking  of  the  finger, 
little  energy  is  expended;  the  sudden  jerking  away  of  the  hand  involves 
many  times  as  much.  Somewhere  this  energy  must  have  been  released 
and  applied;  and  this  is  one  reaction  of  the  series,  whose  final  one  was 
movement.  So  in  the  plant,  stimulation  often  involves  a  mere  fraction 
of  the  energy  expended  in  the  final  movement;  it  is  released,  presumably 
from  the  protoplasm  or  some  part  of  it  that  is  particularly  unstable,  and 
is  applied  to  the  work.  If  this  be  so,  the  .chemical  changes  (metabolism) 
ought  to  be  different  in  a  stimulated  and  unstimulated  organ. 

This  hypothesis,  however,  has  not  yet  been  verified  experimentally.  Reinvesti- 
gation of  the  one  case  in  which  such  a  result  was  reported  has  produced  a  conflict 
of  evidence. 

Another  hypothesis,  that  stimulation  results  in  molecular  strain  only,  from 
which  there  is  gradual  recovery,  sufficiently  accounts  for  fatigue  (see  next  para- 
graph), but  does  not  account  for  the  disparity  in  energy  between  stimulus  and  re- 
action, the  existence  of  which  its  advocates  merely  ignore  or  deny. 

Fatigue,  tetanus,  and  summation.  —  After  an  organ  is  stimulated  once 
and  the  response  occurs,  the  original  state  is  presently  regained,  and  the 
organ  is  ready  to  respond  again  as  at  first  (fig.  671).  If  several  stimuli 
follow,  each  before  complete  recovery,  the  responses  are  of  less  extent 
than  before.  This  effect  is  described  by  the  term  fatigue,  and  in  many 
cases  the  responses  gradually  become  smaller  and  smaller  until  they 


GROWTH    AND    MOVEMENT 


433 


cease  entirely.  When  the  stimuli  recur  very  frequently,  the  responses 
become  f<>r  a  time  combined,  so  thai  the  organ  assumes  a  fixed  position 
unlike  the  unstimulated  <>nc.  This  quite  resembles  the  condition  of  a 
mil  i  le  in  tetanus,  as  can  be  seen  by  comparing  the  records  in  fig.  672. 
After  a  period  of  tetanus,  however,  the  reactions  (case  until  rest  from 
excitation  permits  recovery.     If  stimulation,  too  brief    to  produce  the 


Fig.  671. —  Uniform  electrical  response  in  radish  to  repeated  stimulation.  —  After  Bose. 

end  reaction,  he  repeated  at  proper  intervals,  the  separate  effects  be- 
come combined  and   suffice  presently  to  call  forth  the  end  reaction. 

This  summation  of  stimulation  seems  to  be  a  sort  of  tetanic  piling  up  of 
the  earlier  excitations  of  the  series,  which  finally  becomes  sufficient  to 
transmit  its  effects  to  the  active  region. 


rn^rx 


Fig.  672. 


Records  of  tetanic  contraction  in  muscle  (<;,  b)  and  in  style  of  Datura  (c,  d): 
a,  c,  incomplete;  b,  d,  more  complete.  —  After  Bose. 


Reaction  time.  —  Some  time  elapses  between  the  beginning  of  stimu- 
lation and  the  end  reaction,  and  this  is  appropriately  called  reaction 
time.  Whereas  in  animals  this  is  usually  measured  by  a  fraction  of  a 
-cioiid,  in  plant-  it  is  much  longer,  occasionally  a  few  seconds,  but  often 
minutes  or  even  hours.  This  tardiness  is  due  not  so  mm  li  to  a  low 
degree  of  sensitiveness,  for  the  first  reaction  (perception)  take-  place 
almost  instantly,  a-  to  -low  propagation  and  especially  to  the  sluggish- 
ness of  the  met  hanism  ol  growth.  By  contrast,  turgor  mechanisms  usu- 
ally re-pond  quickly.  Naturally  the  reaction  time  i-  made  up  of  the 
perception  time  (a  -mall  fraction  of  a  second),  the  transmission  time  (the 
rate  varies  commonly  from  o  to  4  cm.  per  second),  and  the  growth  time, 
which  is  far  the  greater  part  of  the  whole  period. 

C.  B.  &  C.   BOTANY  —  28 


434  PHYSIOLOGY 

Presentation  time.  —  In  order  to  produce  any  reaction  a  stimulus  of 
given  intensity  must  act  for  a  definite  time,  called  the  presentation  time. 
For  the  primary  reaction  this  is  extremely  brief  —  practically  instan- 
taneous. But  end  reactions,  especially  those  due  to  growth,  require 
some  minutes  or  even  an  hour  or  more.  Thus,  roots  must  be  kept  hori- 
zontal for  15-30  minutes  or  even  longer  (depending  upon  the  plant  and 
its  condition),  in  order  that  gravity  may  cause  a  curvature.  This  means, 
apparently,  that  the  excitation  must  reach  a  given  pitch  through  con- 
tinuous or  summated  stimulation,  before  it  can  be  propagated  to  the 
active  region  and  affect  the  growth  mechanism.  Once  that  pitch  is 
attained,  the  end  reaction  will  follow;  and  if  the  initial  stimulus  cease 
to  act,  it  will  follow  as  an  after  effect.  If  the  intensity  of  the  stimulus  be 
increased,  presentation  time  is  correspondingly  shortened  (within  limits, 
the  ratio  is  inverse). 

Excitability.  —  To  obtain  a  reaction  it  is  not  enough  that  a  stimulus 
act  upon  a  plant.  The  protoplasm  must  be  in  a  certain  condition,  or 
excitation  cannot  follow.  This  is  clearly  recognized  when  it  is  said  that 
a  "  dead  "  plant  no  longer  responds  to  stimulation  as  before.  It  was 
once  said:  "  The  dead  organism  is  '  dead  '  merely  because  it  has  lost  its 
irritability;  "  but  this  is  true  only  by  an  extension  of  the  term  irritability 
beyond  its  usual  sense.  Closer  study  reveals  the  fact  that  many  agents 
that  do  not  produce  death  temporarily  abolish  or  reduce  or  even  exalt 
excitability.  When  protoplasm  is  in  a  condition  of  excitability,  it  is 
also  in  a  condition  to  carry  on  well  its  usual  activities;  irritability  there- 
fore is  associated  with  other  normal  physiological  qualities  covered  by 
the  term  lone.  One  experiences  the  feeling  of  well-being  and  vigor ;  it 
comes  when  all  the  functions  of  the  body  are  proceeding  properly. 
So  under  favorable  conditions  the  plant's  functions  are  all  effective  and 
this  tonic  condition  may  be  assumed  as  the  norm,1  the  result  of  the  com- 
bined responses  to  many  simultaneous  external  and  internal  stimuli. 
Retardation  or  acceleration  of  particular  functions  may  then  be  brought 
about  by  the  intensification  or  weakening  of  particular  stimuli  of  this 
complex,  or  by  the  application  of  unusual  ones. 

Loss  of  irritability.  —  Excitability  may  be  diminished  or  abolished 
temporarily  by  a  dose  of  anesthetics,  like  chloroform  and  ether,  certain 
other  functions  being  also  interfered  with.  The  precise  mode  of  action 
is  not  known.     After  a  time  the  effect  passes  away  and  tonic  irritability 

1  Note  that  this  is  not  a  fixed  or  well-defined  condition;  it  is  merely  the  usual,  the  ordi- 
nary; and  it  is  assumed  purely  for  convenience. 


GROWTH    AND    MOVEMENT 


435 


is  regained.  By  a  larger  dost'  irritability  may  be  permanently  abolished 
(that  is,  it  kill:-*),  while  by  a  smaller  dose  it  may  become  heightened. 
Various  narcotics  act  in  a  similar  way.  Substances  that  kill  are  usually 
called  poisons;  really  they  are  poisons  only  in  certain  doses.  Their  modes 
of  action  are  doubtless  as  different  as  the  poisons  themselves. 

In  the  following  sections,  the  foregoing  general  principles  will  find  specific 
illustrations  in  the  movements  of  locomotion,  in  the  nastic  ami  tropic  <  urvatures 
of  various  organs,  in  the  displacement  of  leaves  by  motor  organs,  and  in  the  effects 
of  stimuli  upon  form.  It  is  important  that  the  principles  just  set  forth  be  constantly 
referred  to  and  kept  in  mind  in  reading  these  sections. 

3.    MORPHOGENIC    STIMULI 

The  most  general  fashion  in  which  various  external  agents  affect 
growth  appears  in  the  way  they  control  the  form  of  the  body  through  local 
alterations  in  the  development  of  various  parts.  The  varied  and  diffuse 
stimuli  are  termed  formative  or  morphogenic.  The  reactions  to  them  are 
extremely  difficult  to  study  because  both  stimuli  and  reactions  are  so 
general,  and  particularly  because  experimental  alteration  of  one  factor 
is  almost  certain  to  alter  others  to  an  unsuspected  or  an  uncontrollable 
extent;  wherefore  the  analysis  of  the  factors  operating  is  rendered  very 
uncertain.  It  will  be  possible,  therefore,  to  mention  here  only  the 
simpler  and  best  attested  examples. 

Light  and  growth.  —  It  is  well  known  that  the  rate  of  growth  rises  and 
falls  with  the  temperature,  and  since  heat  and  light  are  both  forms  of 


6 
4 

1 

T-,r,, 

674 

L 

t— 

3 

1 

Figs.  673,  674.  —  Graphs  showing  growth  in  millimeters  in  alternating  periods  of  dark- 
ness (shaded)  and  light:  673,  sporangiophore  <>f  Mueor  Mucedo,  periods  15  minutes;  ''74. 

rhi/.oiiis  of  Marcluinlia  polymorpha,  periods  20  minutes. —  Based  on  data  by  Si  ameroff. 


radiant  energy,  it  might  be  expected  that  the  shorter  and  faster  light 
waves  wmild  also  affect  the  rate  of  growth.  This  proves  to  be  true. 
In  general  the  effect  of  light  is  to  retard  growth,  particularly  in  elongating 


436 


PHYSIOLOGY 


organs.  This  is  very  clearly  seen  in  the  sporangiophores  of  Mucor  and 
the  rhizoids  of  Marchantia,  as  will  appear  from  the  graphic  representa- 
tion of  the  observations  (figs.  673,  674).  It  comes  out  also  in  the  auto- 
graphic records  of  the  growth  of  elongating  stems  when  plotted  so 
as  to  show  the  increment  during  the  day  and  during  the  night,  the 
temperature  and  other  conditions,  of  course,  being  kept  as  constant 
as  practicable. 

Daily  period.  —  In  nature  the  retardation  due  to  light  is  doubtless 
accentuated  by  the  greater  evaporation  of  the  daytime;  but  it  is  more 


' 

, 

/ 

V 

\ 

/\ 

/ 

\ 

\ 

I 

\ 

\  / 

\ 

1  * 

/ 

\ 

,..-->''"■- 

•-..     \ 

^" 

■*\ 

V 

7 
6 

5 
4 

3 

2 

t 

mmL 


60 

T 

F° 

8  10  Mm.  2  4  6  8  10  12;/.  2  4  6  8  10 
Fig.  675.  — Curve  of  daily  period  (solid  line)  and  of  temperature  (broken  line) :  each 
vertic  al  interval  corresponds  to  i  mm.  increment  for  the  growth  curve  and  to  50  F.  for  the 
temperature  curve;  horizontal  intervals  are  hours,  the  region  of  close-set  lines  showing 
the  night.  Note  rapid  growth  during  the  first  day,  beginning  to  fall  off  before  the  tem- 
perature falls  (probably  a  transpiration  effect)  and  then  rising  in  the  night  in  spite  of 
falling  temperature  (partly  also  a  moisture  effect).  —  From  data  by  Spoehr. 


or  less  compensated  by  the  acceleration  due  to  the  rising  temperature. 
Contrariwise,  the  acceleration  upon  the  coming  of  darkness  and  a  moister 
air  is  partly  offset  by  the  retardation  due  to  the  lower  temperature  of  the 
night.  Nevertheless,  a  periodic  variation  in  growth  in  length,  corre- 
sponding to  the  day  and  night,  and  hence  called  the  daily  period,  can 
be  traced,  unless  the  fluctuations  of  temperature  are  excessive.  This 
means  that  as  certain  conditions  act  antagonistically  upon  the  rate  of 
growth,  they  may  be  balanced  or  one  set  may  overcome  the  other.  The 
difference  between  the  darkness  of  night  and  the  light  of  day  is  so  much 
greater  than  the  usual  differences  of  temperature  and  moisture  in  these 
hours,  that  the  light  effect  is  likely  to  be  dominant  (fig.  675). 


GROWTH    AND   MOVEMENT 


437 


Light  and  form.  —  The  form  of  the  aerial  part--  of  most  plant-  is  pro- 
foundly influenced  by  light,  dim  dyor  indirectiy.  This  is  shown  by  the 
striking  changes  that  ensue  {etiolation)  when  they  are 
grown  in  darkness.  Without  starvation  this  is  pos- 
sible only  with  plants  that  have  already  stored  a 
sufficient  amount  of  surplus  food.  One  who  has 
observed  the  long  pallid  shoots  of  a  potato  which 
has  sprouted  in  the  dark  will  have  seen  the  general 
effects.  The  stems  tend  to  elongate  much  more  than 
usual,  though  they  are  not  necessarily  more  slender; 
the  branching  is  at  a  different  angle;  and  the  leaves 
remain  small  and  imperfectly  developed.  (The  pallor 
from  lack  of  chlorophyll  and  the  presence  of  carotin 
are  features  already  mentioned.)  On  the  whole, 
elongation  is  likely  to  be  accentuated,  breadth  is  likely 
to  be  repressed  (fig.  676).  Though  these  are  the 
common  results  of  the  lack  of  light  during  develop- 
ment, they  are  by  no  means  universal.  Thus,  there 
are  plants  whose  stems  do  not  elongate,  and  others 
whose  leaves  arc  not  reduced.  But  if  not  these, 
other  characteristics  may  be  altered;  e.g.  reduction 
of  the  mechanical  elements  of  the  tissues  is  one  of 
the  less  obvious  effects.  Scarcely  a  plant  escapes  but 
those  that  pass  all  their  lives  in  darkness,  and  only 
those  parts  that  are  buried  in  the  soil  are  exempt  from 
the  formative  influence  of  light. 

Dorsiventrality.  —  In  plant  organs  not  grown  in 
darkness,  but  of  which  one  side  is  better  illumi- 
nated than  the  other,  light  effects  can  be  observed. 
One  effect  is  the  development  of  a  distinctly  different 
structure  in  the  better  lighted  surface  ;is  compared 
with  the  shaded  one,  and  since  these  are  naturally  the 
upper  and  under  surface-,  an  organ  showing  such 
difference-  is  termed  dorsiventrol.1  Thus  the  pali 
sade  portion  of  the  mesophyll  of  leaves  owes  its  exist- 
ence chiefly  to  light.2  Dorsiventrality  in  the  liverworts  is  likewise  due 
mainly  to  light.     None  shows  this  better  than  the  common  Man  Jnwtiii  t 


l  re  676.— Plant 
of  Phaseolus  grown 
in  darkness.  -  AiU  r 
MacDoogal. 


1  Dorsiventral  organs  may  owe  the  difference  of  their  fai  es  t<>  other  formative  stimuli, 
■  g.  to  gravity.  Se<  footnote,  p.  4a  1. 


438  PHYSIOLOGY 

If  a  gemma  (p.  98),  which  when  separated  from  the  parent  is  just 
alike  on  the  two  sides,  be  grown  in  a  moist  chamber  with  the  lower 
side  illuminated  and  the  upper  dark,  air  chambers  will  be  developed 
on  the  lighted  side  and  rhizoids  on  the  dark  one,  exactly  the  reverse 
of  the  usual  relation.  Gravity,  if  it  furnish  any  stimulus,  as  is  prob- 
able, is  clearly  overcome  by  light.  In  like  manner  light  determines 
the  formation  of  the  sex  organs  upon  the  under  side  of  fern  prothallia. 
A  striking  example  of  light  effects  among  the  seed  plants  is  to  be  found 
in  the  dorsiventrality  of  the  rootstocks  of  the  spatter  dock  (Nympheca 
advene).  These  great  rhizomes  develop  at  the  surface  of  the  mud  at 
the  bottom  of  pools,  and  are  of  the  length  and  thickness  of  a  man's 
arm.  From  the  upper  side  numerous  leaves  arise,  and  from  the  under 
side  roots.  This  distribution  of  organs  is  found  to  be  determined  by 
differences  in  lighting. 

Electric  waves.  —  Of  the  same  class  as  heat  and  light  waves  are  the 
electric  waves;  and  they  too  have  considerable  formative  influence.  It 
has  been  shown  that  the  germination  of  many  seeds  is  hastened  by  suit- 
able electric  stimuli,  and  for  a  considerable  time  the  growth  of  seedlings 
is  also  accelerated.  When  crops  of  barley,  wheat,  beets,  and  other 
economic  plants  are  frequently  subjected  to  a  quiet  discharge  of  high- 
tension  currents  from  wires,  with  many  pendent  points,  strung  over  the 
experimental  fields,  it  has  been  found  by  several  observers  that  the 
plants  grow  better,  come  to  maturity  earlier,  show  increased  productiv- 
ity, and  are  of  better  quality  than  on  control  plots. 

Thus,  an  electrified  wheat  plot  of  3  acres  yielded  a  crop  39  per  cent  greater  than 
the  control  plot,  sold  at  7.5  per  cent  higher  prices,  and  the  flour  was  of  a  higher 
grade  on  account  of  its  baking  quality.  Beets  (for  the  table)  on  an  electrified  plot 
showed  ^^  per  cent  increase  and  contained  an  average  of  8.8  per  cent  sugar,  against 
7.7  per  cent  on  the  control  plot. 

Chemical  agents.  —  Chemical  stimuli  are  also  extremely  important 
in  determining  the  form  of  plants.  The  presence  or  absence  of  particular 
substances  in  the  cells,  whether  foods  or  wastes,  doubtless  exerts  a  pro- 
found influence.  But  the  precise  influence  of  the  different  compounds 
cannot  be  determined  satisfactorily,  because  the  chemical  processes 
within  the  plant  are  so  imperfectly  known.  It  is  in  this  region  that  the 
role  of  the  so-called  necessary  elements  of  the  ash,  calcium,  magnesium, 
potassium,  and  iron  are  to  be  sought,  in  all  probability.  How  far  the 
xerophytic  structure  of  plants  is  to  be  ascribed  to  the  lack  of  water  is  not 
certain.     The  deficiency  of  available  water  may  be  in  itself  a  chemical 


GROWTH    AND   MOVEMENT 


439 


stimulus,  or  it  may  make  possible  the  stimulating  action  of  other  sub- 
stances within  the  plant,  which,  but  for  their  increased  concentration, 
would  not  act  so.  Unquestionably  other  causes  than  lack  of  water 
around  the  roots  of  a  plant  may  call  forth  such  structures,  as  is  well  seen 
in  the  case  of  bog  plants.  Indeed  it  has  become  customary  to  speak  of 
"physiological"  drought  as  the  cause  of  serophytic  structure,  when 
physical  drought  is  obviously  out  of  the  question.  This  may  be  taken 
as  a  convenient  expression  for  some  difficulty  which  prevents  the  plant 
from  admitting  a  sufficient  amount  of  water,  such  as  the  poor  develop- 
ment of  the  root  system.  Whatever  does  this  will  tend  to  dwarf  or  other- 
wise transform  the  aerial  parts,  either  as  the  plain  lack  of  water  does,  or 
possibly  in  quite  different  and  unrecognized  ways.  (See  further,  Part 
III  on  dwarfing  in  bogs.) 

Recent  investigations  are  bringing  to  light  some  new  causes  for  the  imperfect 
development  of  plants,  which  probably  is  due  primarily  to  an  effect  on  the  roots. 
It  is  found  that  the  sterility  of  some  soils  is  due  to  the  presence  in  these  soils  of  organic 
substances,  which  are  partly  soluble,  so  that  a  watery  extract  of  such  soils,  when 
used  as  a  water-culture  medium,  acts  as  badly  as  the  soil  itself.  Furthermore, 
these  substances  can  be  removed  in  large  part  by  adding  some  finely  divided  ma- 
terial like  lampblack  to  the  liquid  and  then  filtering  it  out.  The  filtrate  may  then  be 
used  without  detriment  to  the  cultures.  Still  further  study  makes  it  probable  that 
these  substances  originate  in  large  part  from  the  plants  which  have  previously  grown 
in  the  soil.  The  necessity  for  the  rotation  of  crops  on  any  field  has  long  been  known. 
The  reason  has  been  assumed  to  lie  in  the  exhaustion  of  the  materials  which  are 
supposed  ti>  be  nei  essary  fur  the  nutrition  of  the  plants.  Without  denying  that  there 
may  be  something  in  this  assumption  (it  is  nothing  else  at  present,  because  the  ex- 
perimental evidence  upon  which  it  rests  is  faulty),  it  seems  now  much  more  likely 
that  the  chief  cause  is  to  be  found  in  the  excretions  from  the  roots  of  the  previous 
crops  and  the  products  of  their  decay  in  the  soil.  It  has  been  shown  that  though 
the  mineral  salts  of  a  culture  solution  be  maintained  unchanged,  the  water  becomes 
more  and  more  unfit  for  use  with  repeated  cultures  of  the  same  species,  and  that 
this  impairment  may  be  remedied  by  treatment  with  lampblack  as  above  desc  ribed, 
though  the  content  of  salts  be  not  altered.  Water  cultures,  to  which  have  been 
added  various  orgai  ic  substances  that  might  be  produced,  or  are  known  to  occur 
in  plants,  have  shown  like  injuries  to  the  plants,  and  though  the  amount  of  the 
deleterious  sulistam :es  occurring  in  nature  is  too  small  for  direct  analysis,  their 
general  i  harm  ter  may  be  ascertained  by  further  experimentation  in  this  way. 

Mechanical  agents.  —  Pressure  and  tension  have  evident  influence 
on  the  development  of  mechanical  tissues.  The  encasing  of  a  stem  in 
a  plaster  cast,  SO  that  as  it  thickens  it  will  compress  itself,  leads  to 
changes  in  the  stru<  ture  within  the  zone  of  compression  and  especially 
just  beyond  the  margin.     Continuous  tension  seems  to  bring  little  if  any 


440  PHYSIOLOGY 

increase  in  mechanical  tissues,  but  ilexure,  with  its  alternating  compres- 
sion and  tension,  such  as  the  wind  in  certain  regions  produces,  beyond 
doubt  increases  the  proportion  of  mechanical  tissues  and  thickens  their 
walls.  When  combined  with  excessive  evaporation  and  perhaps  other 
unfavorable  factors,  the  effect  on  bodily  form  is  astonishing  (see  Part  III 
on  stem-dwarfing). 

Deformities.  —  Noteworthy  local  modifications  of  form  are  produced 
by  the  attacks  of  parasites,  either  plant  or  animal.  When  specific 
deformities  are  produced,  the  structures  are  called  galls  (fig.  655,  p.  384). 
Just  how  far  these  are  due  to  chemical  substances  excreted  by  the  para- 
site, and  how  far  to  the  mechanical  pressure,  to  the  punctures,  or  to  the 
movements  of  the  larvae  of  animal  parasites,  remains  at  present  quite 
uncertain.  Whether  chemical  or  mechanical  stimuli  act  upon  the  host, 
its  response  might  be  first  an  altered  metabolism,  which  produces  ap- 
propriate effects  upon  the  division  and  course  of  development  of  the 
cells,  resulting  in  the  deformation  of  the  region.  Profound  alterations 
in  the  relative  development  of  the  tissues  and  in  the  character  of  their 
elements  accompany  the  deformity. 

Injuries.  —  Injuries  of  various  sorts  call  forth  growth  in  tissues  which 
have  long  passed  the  ordinary  period  of  cell-division.  This  gives  rise  to 
a  callus  at  the  edges  of  the  wound  which  tends  to  close  it,  a  fact  that  is  of 
great  practical  service  in  the  grafting  and  budding  so  indispensable  in 
fruit  growing.  Desirable  sorts,  too  tender  for  a  given  climate,  may 
thus  be  united  with  stocks  that  are  hardy,  but  have  no  good  qualities 
in  their  fruit. 

In  practice,  smoothly  cut  surfaces  are  opposed  and  kept  in  close  contact,  with  the 
exclusion  of  water  and  spores  by  wrappings  and  wax.  The  healing  tissues  blend, 
as  they  form  at  the  junction,  and  an  organic  union  is  established,  permitting  the 
passage  of  water  and  foods  freely. 

If  a  wound  be  allowed  to  heal,  the  callus  may  give  rise  to  new  growing 
points,  from  which  the  regeneration  of  removed  organs  may  proceed. 
Thus,  if  a  root  be  decapitated,  a  new  apex  may  be  regenerated,  if  the 
cut  be  near  enough  the  tip,  or  new  lateral  roots  may  arise  that  would 
not  otherwise  have  been  produced,  or  old  roots  may  be  incited  to  more 
active  growth.  In  either  case  of  the  formation  of  new  organs,  the  reac- 
tion to  the  wound  stimulus  is  complicated  with  unknown  factors  named 
polarity,  and  with  the  influence  of  other  organs  called  correlations. 

Polarity.  —  Since  the  opposite  ends  of  an  egg  cell  give  rise  to  unlike 
structures  (for  example,  in  seed  plants,  suspensor  cells  from  one  end  and 


GROWTH    AND    MOVEMENT  441 

the  embryo  initial  from  the  other),  it  is  assumed  that  the  two  hemi- 
spheres arc  unlike,  oven  though  no  structural  differences  arc  visible. 
This  is  expressed  by  the  term  polarity,  after  the  analogy  of  the  invisible 
differences  in  the  two  ends  or  poles  of  the  magnet.  A  like-  polarity  must 
be  imputed  to  all  other  cells,  its  progeny,  so  that  the  embryo  initial,  when 
it  develops,  produces  at  the  one  cud  a  rool  and  at  the  other  a  shoot. 
Later  in  life,  any  piece  of  the  shoot  cut  away  from  the  rest  shows  a  ten- 
den<  y  to  produce  shoots  at  the  apical  end  and  routs  at  the  basal  end,  when 
put  under  conditions  to  regenerate  lost  organs.  The  conception  of 
polarity  in  the  cells  is  thus  extended  to  aggregates  of  cells  of  any  size. 
because  they  show  such  differences  at  the  apical  and  basal  ends.  All 
attempts  to  ascertain  the  nature  of  polarity  have  so  far  proved  futile, 
so  that  there  is  nothing  to  "  explain  "  the  phenomena  but  the  word  and 
the  assumption  for  which  it  stands. 

Correlations.  —  The  term  correlation  designates  the  reciprocal  influ- 
ence of  organs.  Of  this  little  is  known  beyond  the  fact  that  the  suppres- 
sion or  the  removal  of  one  organ  exercises  a  marked  effect  upon  some  or 
all  of  the  remaining  ones.  Many  examples  might  be  cited,  but  no  ade- 
quate explanation  of  the  effects  can  be  given.  It  is  known,  however, 
that  at  least  some  of  them  are  not  due  merely  to  differences  in  the  InA 
or  water  supply,  or  to  like  conditions.  Examples  will  make  clear  what 
is  meant  by  correlations. 

Quantitative  correlations.  —  In  the  axil  of  each  cotyledon  of  the  bean  there  is 
present  a  bud,  neither  of  which  develops  into  a  shoot  unless  the  main  axis  is  cut 
off  or  prevented  from  developing.  If  one  desires  sweet  peas  and  such  plants  to 
continue  flowering,  it  is  necessary  to  cut  away  the  older  flowers  or  the  youm,'  pods, 
so  as  to  prevent  the  formation  of  fruit.  If  this  is  done,  the  plants  go  on  flowering 
till  frost,  whereas  their  season  is  quickly  over  when  allowed  to  sel  seed.  The 
gametophyte  of  ferns  is  shortdived,  as  a  rule;  but  if  the  fertilization  of  the  egg 
be  prevented,  its  life  may  he  prolonged  for  months,  and  it  proliferates,  forming 
nil  again  and  again.  The  possibility  of  shaping  a  tree  by  judicious  prun- 
ing, and  of  increasing  the  production  of  fruits  by  ore  hard  trees  in  the  same  way 
rests  upon  like  reactions. 

Qualitative  correlations.  — Correlations  are  not  merely  quantitative,  as  the  above 
examples  might  seem  to  imply;  they  are  also  qualitative.  That  is,  the  whole  be- 
havior and  even  the  structure  of  an  organ  may  he  altered  according  as  other  organs 

are  present  or  absent.  Thus,  the  central  axis  of  most  conifers  is  strictly  radial  in 
structure  and  in  branching,  while  the  lateral  branches  are  distinctly  dorsiventral. 
But  if  the  terminal  shoot  be  cut  away,  one  (<>r  more)  of  the  laterals  may  become 
erei  t,  losing  entirely  the  dorsi ventralit y,  and  becoming  radial  like  the  leader.     The 

aerial  shoots  of  the  potato,  which  hear  foliage  leaves  and  Bo  vers,  are  very  different 
from  the  subterranean  ones,  which  hear  the  scales  and  tubers.      But  if  the  aerial 


442  PHYSIOLOGY 

shoots  be  cut  away,  some  of  the  subterranean  shoots  will  (urn  up  into  the  air,  be- 
come green,  and  develop  foliage  and  flowers  as  though  never  inclined  to  be  subter- 
ranean. The  sporophylls  of  certain  ferns,  notably  Onoclea,  arc  entirely  different  in 
aspect  from  the  nutritive  leaves,  and  have  so  many  sporangia  crowded  on  the  sur- 
face that  they  seem  entirely  covered.  If  all  the  nutritive  leaves  be  cut  away,  leaves 
that  ordinarily  would  have  become  sporophylls  will  then  becomo  foliage  leaves 
and  bear  no  sporangia.  In  like  manner  the  tendrils  of  the  pea  leaf  may  be  made  to 
develop  into  leaflets. 

In  all  these  cases  transformation  is  possible  only  before  the  primordia 
have  gone  too  far  in  any  determined  course,  though  the  point  at  which 
new  influences  may  affect  them  is  very  different  in  the  different  cases. 
Usually  the  stimulus  must  be  applied  very  early,  while  the  primordia  are 
still  undifferentiated.  Many  of  the  problems  of  regeneration  are  com- 
plicated by  these  phenomena  of  correlation,  if  they  are  not  wholly  de- 
termined by  them. 

4.   NASTIC   CURVATURES 

Epinasty  and  hyponasty.  —  A  somewhat  less  general  manner  in  which 
stimuli  of  various  sorts  affect  plants  is  to  be  found  in  their  effects  upon  the 
rate  of  growth  on  the  two  faces  of  bilateral  organs,  such  as  thalli,  foliage 
leaves,  bud  scales,  perianth  leaves,  etc.  It  is  very  common  to  find  that 
such  organs  grow  at  different  rates  on  the  two  faces,  so  that  they  are 
distinctly  curved  thereby.  Thus,  in  their  earliest  stages,  the  leaves 
grow  fastest  on  the  back  or  outer  side,  so  that  the  inner  face  is  pressed 
close  to  the  axis,  and  as  they  usually  outgrow  it,  they  curve  together  over 
it  in  a  protective  fashion,  forming  a  bud.  The  scales,  especially,  long 
maintain  this  form,  as  the  longitudinal  section  of  any  bud  will  show. 
Later,  the  relative  rate  changes;  the  inner  face  grows  more  rapidly  than 
the  outer,  and  the  bud  opens  because  the  curvature  carries  the  leaf  or 
scale  away  from  the  axis.  Thalli  often  show  the  same  thing;  the  upper 
surface  may  be  so  tense  from  greater  growth  that  the  thallus  is  tightly 
appressed  to  the  ground.  Such  curvatures  are  described  briefly  by  the 
terms  epinasty  or  hyponasty,  according  as  the  greater  growth  is  on  the 
upper  (inner)  or  lower  (outer)  face.  The  greater  number  of  these  nastic 
curvatures  are  due  to  unknown  (internal?)  causes,  but  some  have  been 
found  to  be  reactions  to  external  stimuli  (paratonic).  The  former  are 
not  unlike  those  autonomic  curvatures  of  radial  organs  described  as 
nutations  (p.  423),  only  in  this  case  the  bilateral  structure  of  the  organ 
determines  that  the  nutations  shall  be  in  one  plane  only.  The  latter 
are  also  allied  to  tropisms,  but  differ  from  them  in  that  net  the  direction 


GROWTH    AND   Ml  >VEMENT 


443 


from  whi<  li  (lie  stimulus  acts  but  the  structure  of  the  organ  predetermines 
the  plane  of  the  movement. 

Light  and  temperature.  —  Examples  of  paralonic  nastic  curvature  are 
seen  when  light  and  temperature  act  as  stimuli  upon  foliage  and  flower 
leaves,  and  less  plainly  in  tendrils.  Temperature  changes  arc  espo  ially 
effective  with  the  perianth  leaves  of  tulip,  crocus,  snowdrop,  colchi<  um, 
and  other  plants  whose  blossoms  appear  very  late  in  the  autumn  or  very 
early  in  the  spring.  In  the  crocus  a  rise  of  half  a  degree  suffices  to  bring 
about  a  curvature  that  opens  the  flower;  while  the  tulip  can  be  made 
to  open  and  close  as  many  as  eight  times  in  the  course  of  an  hour  by 
raising  and  lowering  the  temperature.  Tendrils  respond  to  a  tempera- 
ture change,  whether  a  rise  or  a  fall,  by  curving  in  one  direction  only, 
the  upper  side  being  stimulated  to  accelerated  growth.  In  this  they  differ 
from  the  perianth  leaves  cited,  for  in  these  a  rise  of  temperature  tends 
to  accelerate  growth  on  the  inner  face  and  thus  to  open  the  flower,  and 
a  fall  to  accelerate  growth  of  the  outer  face  and  so  to  close  the  flower. 
Very  many,  perhaps  the  majority  of  foliage  leaves,  show  nastic  curvatures 
in  response  to  alterations  in  temperature  and  light  as  long  as  the  petiole 
is  still  capable  of  growing;  finally  curvature  ceases  &&  tfae  fixed  light 
position  of  maturity  is  attained.  Such  bending  movefn^n© remind  one 
of  the  photeolic  movements  executed  throughout  life  by  le^jes  that  have 
motor  organs  (see  p.  451).  Among  flowers  those  nest  ^trikingly  re- 
sponsive to  light  are  the  heads  of  some  Compositae,  s^h  as  the  dande- 
lion. Here  the  flowers  and  the  bracts  about  the  flfT^rgJ-luster,  the 
involucre,  curve  so  as  to  close  the  head  when  the  light S^dminished,  as 
in  cloudy  days,  and  to  open  it  in  sunshine  (Part  III,  fiq£i&,  1194). 

In  countries  where  the  climate  is  equable  it  is  possible  to  select  plants  whose 
Bowers  open  at  particular  hours  of  the  day  on  account  of  light  and  temperature 
stimuli,  and  by  planting  them  in  a  circle  to  have  a  sort  of  floral  clock.  Naturally 
it  is  not  very  reliable. 

Gravity.  —  Nastic  curvatures  are  also  produced  in  plants  in  response 
to  gravity,  which,  however,  usually  cooperates  with  or  antagonizes  the 
light  reactions.  In  all  cases  the  stimulus  at  work  may  be  indicated  by 
(he  nrvlix.  Thus  we  have  photonasty,  thermonasty,  etc.,  and  still  more 
specifically  photepinasty,  geohyponasty,  etc. 

Mechanism.  —  In  all  these  curvatures  the  mechanism  of  response  is 
the  same.  The  growth  of  the  outer  or  inner  surface  i-  accelerated,  as 
can  be  shown  by  making  equidistant  marks  upon  the  two  faces  and  mea- 
suring the  changes.     This  observation  shows,  too,  that  under  frequent 


444 


PHYSIOLOGY 


stimulation  the  total  growth  is  much  greater  than  it  is  under  uniform 
conditions. 

5.  LOCOMOTION  AND  STREAMING 

Locomotion  limited.  —  Locomotion  is  restricted  among  plants  to  the 
simplest  forms  (with  a  few  exceptions  to  those  that  are  unicellular), 
and  to  the  gametes,  especially  the  male  gametes,  of  the  multicellular 
plants.  The  reason  for  this  is  doubtless  to  be  found  in  the  restriction  of 
freedom  to  move  imposed  by  the  cell  wall  —  in  effect  a  sort  of  strait- 
jacket  —  in  which  the  protoplast  incases  itself.  Even  when  the  proto- 
plast moves,  as  it  often  does,  within  this  case, 
its  movements  do  not  bear  against  the  outer 
medium  and  therefore  do  not  propel  it  about. 
The  only  exception  to  this  restriction  occurs 
in  those  plants  whose  wall  is  perforate;  then 
the  protoplasm  protrudes  through  the  opening 
so  as  to  operate  against  the  outer  medium,  or 
in  a  few  cases  it  excretes  mucilage  forcibly 
against  the  medium  or  the  substratum  and  so 
pushes  itself  slowly  along. 

Rate.  —  When  the  protoplast  changes  its 
shape  suddenly,  quick  swimming  and  darting 
movements  result  ;  when  slowly,  the  move- 
ment is  perceptible  only  because  magnified  by 
the  microscope.  In  the  very  swiftest  move- 
ments the  absolute  translation  is  small,  say  50  mm.  per  minute;  and 
in  the  sperms  of  ferns,  which  under  the  microscope  seem  to  be  going 
fast,  the  rate  is  only  0.1  to  0.2  mm.  per  minute.  Measured  relatively, 
as  in  terms  of  size,  and  taking  account  of  the  resistance  of  the  medium, 
the  translation  is  seen  to  be  very  rapid.  The  very  fast  human  runners 
cover  about  50  times  their  own  length  (100  yards)  in  10  seconds;  the 
swarm  spores  of  Viva  can  travel  100  times  their  own  length  in  the 
same  time;  and  the  spiral  sperms  of  a  fern  (N ephrodiiim)  can  do  50  to 
100  times  their  length  (as  coiled)  in  10  seconds  (fig.  677). 

Amoeboid  movements.  —  The  slow  movements  are  a  kind  of  creeping, 
and  are  of  two  sorts,  amoeboid  and  excretory.  Amoeboid  movements 
(so  called  because  characteristic  of  Amoeba,  a  genus  of  infusoria)  are 
found  rarely  among  plants,  being  known  only  in  the  plasmodia  of  Myxo- 
mycetes,  a  group  of  organisms  with  so  many  animal  characters  that  they 


Fig.  677.  —  Sperm  of  Ne- 
phrodium,  with  flagella 
—  After  Yamanouchi. 


GROWTH    AND    MOVEMENT 


445 


arc  often  included  in  the  animal  kingdom  (see  p.  i).  The  plasmodium 
is  a  naked  mass  of  protoplasm  (sometimes  like*  a  thin  cake,  often  a  richly 
anastomosed  network),  which  during  its  vegetative  period  lives  in  wet 
places  among  decaying  wood,  leaves,  etc.  The  creeping  is  a<  complished 
by  the  protrusion  of  marginal  lobes  of  the  protoplast  along  one  side,  and 
toward  these  the  rest  slowly  flows.  In  this  way  the  whole  mass  advani  es 
in  a  definite  direction,  which  is  frequently  changed  and  is  subject  to 
control  by  external  agents.  Thus,  by  varying  the  temperature,  the  mois- 
ture, or  the  illumination,  the  plasmodium  may  be  made  to  creep  in  one 
direction  or  another.  Its  response  to  these  stimuli,  however,  differs 
with  its  own  stage  of  development.  Whereas  during  a  considerable 
vegetative  period  it  avoids  light  and  drier  places,  later  it  creeps  out  from 
the  substratum  and  ascends  to  drier  and  exposed  situations,  where  it 
produces  sporangia  with  a  casing  and  framework  of  cellulose  and  a 
multitude  of  spores. 

Excretory  movements.  —  Excretory  movements  are  executed  by  some 
diatoms  and  desmids,  and  those  of  Oscillatoria  and  Spirogyra  are 
probably  of  this  sort.  The  diatoms  and  desmids  forcibly  excrete  muci- 
lage through  slits  or  pores  in  the  wall  against  the  substratum  (a  glass  slide, 
the  wall  of  an  aquarium,  the  bottom  of  a  pool,  or  the  surface  of  a  water 
plant)  over  which  they  creep  slowly  with  a  majestic 
and  mysterious  motion,  which  is  not  yet  fully  under- 
stood (see  also  p.  451). 

Ciliary  movement.  — The  more  rapid  movements 
are  called  ciliary-,  because  executed  by  the  lashing  of 
slender  threads  of  protoplasm  through  the  water,  in 
which  alone  such  organisms  can  move.  The  motile 
threads  are  known  as  cilia  or  flagella.1  They  arise 
from  different  places  on  the  protoplast,  often  at  the 
pointed  apex  or  along  a  band,  where  the  special 
organ  which  produces  them,  the  blepharoplast,  is 
lo<  ated  (fig.  678).  The  flagellates  (unicellular  organ- 
isms of  uncertain  relationship,  p.  20),  bacteria,  the 
zoospores  and  gametes  of  certain  algae  and  fungi, 
and  the  sperms  of  bryophytes,  pteridophytes,  and 


Fig.  678.—  Swarm 
spore  of  Hydrodit  tyon, 
with  two  cilia  ari  ing 
from  a  blepharoplast 
with  nu<  lear  CODDl  l  - 
lions.— After  Timbi  k- 

LAKE. 


1  No  constant  distinction  can  be  made  between  cilia,  which  arc  typically  short,  hair- 
like, and  numerous,  and  Bagella,  which  arc  long,  whiplike,  and  few  (1   4)  for  1 
Yet  a  cell  sometimes  has  a  single  (.ilium,  OT  tWO, and  llayella  are  numerous  on  the  sperms 

of  ferns. 


446  PHYSIOLOGY 

cycads,  exhibit  ciliary  locomotion.  The  cilia  are  so  slender,  and  when 
magnified  sufficiently  their  movements  are  so  rapid,  that  the  details  of 
the  strokes  are  difficult  to  follow.  In  the  thicker  cilia  of  infusoria  the 
forward  stroke  (fig.  679)  consists  of  a  progressive  bending,  which  begins 
below  the  free  tip  and  advances  to  the  base,  where  it  is  most  powerful. 
At  the  moment  of  greatest  efficiency  (fig.  679,  2),  the  curve  bears 
against  the  water  like  the  blade  of  an  exaggerated  spoon  oar  (though, 
of  course,  the  cilium  is  not  flattened).  The  return  stroke  (fig.  680)  is 
slower  and  consists  of  a  reverse  and  some- 
what different  curvature,  advancing  from 
base  to  apex. 

Cause.  —  The  cause  of  these  repeated 
lashings  is  completely  hidden.  They  con- 
tinue for  a  time  and  then  cease.  Though 
they  cannot  be  initiated,  they  can  be 
stopped  or  modified  in  rate  by  appropriate 
stimuli,  and  their  duration  can  be  pro- 
Figs.  679,   680.  —  Diagram-    longed.     Thus,  if   zoospores    of    algae  be 

matic  representation  of  sucessive  rdeased  in  Hght  they  may  swim  about 
positions  (as  numbered)  of  cilia 

of  Urostyla  grandis ;  679,  in  for-  for  a  few  hours,  then  attach  themselves 
ward  stroke;  680,  in  recovery,  and  germinate.  But  if  they  be  kept  in 
-After  Verworn.  darkness,  the  swimming  may  continue  for 

two  or  three  days,  until  the  zoospore  seems  entirely  exhausted  and 
perishes  without  settling  down. 

Taxies.  —  The  direction  of  swimming  may  also  be  controlled  by  ex- 
ternal agents.  The  phenomena  of  directed  locomotion  are  compre- 
hensively called  taxies,  and  with  a  prefix,  designating  the  directive  agent, 
we  have  phototaxy,  thermotaxy,  chemotaxy,  etc.  These  responses, 
apparently  simple,  are  really  very  difficult  to  interpret,  and  experiments, 
seemingly  quite  conclusive,  may  lead  to  false  inferences  through  the 
operation  of  some  overlooked  factor.  Thus,  if  a  dish  containing  zoo- 
spores of  algae  be  placed  on  a  window  ledge  so  that  one  side  is  more 
brightly  illuminated  than  the  other,  the  swarm  spores  will  be  seen  to 
accumulate  on  the  side  with  brighter  light,  and  this  movement  was 
described  at  first  as  a  positive  response  to  light.  Later  it  was  found 
that  the  droplets  in  an  oil  emulsion  would  behave  in  the  same  way 
because  of  the  previously  unnoticed  differences  in  temperature,  making 
convection  currents  in  the  dish.  Two  factors  were  therefore  involved 
and  more  rigid  tests  were  needed  to  demonstrate  phototaxy. 


CROW  III    AM)    Mi  »VI  Ml  A  I 


•I  i; 


Chemotaxy.  —  Chemotaxy  has  been  most  extensively  investigated, 
but  is  not  yet  fully  elucidated.  If  a  soluble  crystal  be  introduced  into 
water  undisturbed  by  currents,  the  molecules  gradually  diffuse  from 
its  surface  in  a  constantly  enlarging  sphere;  or  if  the  water  be  the  film 
under  a  cover  glass,  in  an  increasing  /.one-.  By  using  a  glass  tube  drawn 
out  to  a  very  fine  capillary  and  closed  at  one  end,  liquids  of  any  sort 
may  be  used.  A  short  capillary  is  filled  with  the  solution  and  placed 
on  a  microscope  slide  with  its  open  end  under  the  cover  glass.  Slow 
diffusion  takes  place  from  the  mouth,  while  the  behavior  of  the  organisms 
is  watched  under  the  microscope.  As  a  rule  the  rate  of  their  movement 
is  not  affected,  except  by  substances  that  are  directly  injurious.  It 
appears  that  the  directive  effect  of  such  stimuli  is  exercised  in  two  dif- 
ferent way-. 

i.  Orienting  reaction.  —  In  the  first  case,  the  direction  is  altered 
because  the  organism,  in  response  to  the  stimulation,  orients  itself,  so 
that  with  continued  movement  the  body  will  be  carried  toward  or  away 
from  the  source  of  the  diffusing  molecules.  It  is  assumed  that  this 
orientation  is  determined  by  the  unequal  or  one-sided  action  of  the 
molecules,  the  end  (less  probably  the  flank)  toward  the  source  being  most 
powerfully  affected,  whereupon  the  creature  turns,  and  according  as  it 
brings  the  anterior  or  the  posterior  end  toward  the  source  of  stimulus, 
and  swims,  it  will  approach  or  recede  from  that  source. 

2.  Recoil  reaction.  —  The  second  case  is  quite  different.  The  move- 
ments of  sperms  and  zoospores  are  too  rapid  to  be  followed  easily;  but 
if  large  and  slow-moving  organisms  are  observed,  they  may  be  seen  to 
swim  about  quite  indifferently,  passing  in  close  proximity  to  the  crys- 
tal or  capillary  tube  from  which  the  molecules  are  diffusing,  without 
showing  any  tendency  to  swim  towards  it.  But  when  they  reach  by 
chance  the  limits  of  the  diffusion  /one,  they  suddenly  reverse  their  direc- 
tion and  back  away,  as  though  they  had  encountered  an  obstacle  and 
had  rebounded  from  it.  This  reaction  is  repeated  at  every  side,  and 
having  one  e  chani  ed  to  swim  into  the  diffusion  /.one,  they  are  imprisoned 
within  it,  because  the  attempt  to  pass  out  of  it  results  always  in  the  re^ 
action  of  ret  oil.  So.  ;i^  mure  and  more  an-  thus  caught,  there  is  an 
accumulation  within  the  diffusion  zone,  as  though  it  were  a  trap.  Not 
all  substances,  however,  permit  the  first  accidental  entry,  for  the  recoil 
may  be  produced  at  the  attempt  to  cuter  this  zone,  while  any  such  organ- 
isms placed  within  it  would  be  free  to  swim  out  without  recoil.      In  such 

a  i  ase  the  final  result  is  the  accumulation  of  the  organisms  in  the  regions 


448  PHYSIOLOGY 

outside  the  diffusion  zone.  Besides  the  reaction  of  recoil,  there  are 
accompanying  minor  reactions  which  cannot  be  discussed  here. 

Attraction  and  repulsion.  —  Many  different  substances  have  been 
tested  with  respect  to  chemotactic  control.  Some  prove  to  be  attractive, 
some  indifferent,  and  some  repellent.  That  responses  occur  to  substances 
that  are  never  met  in  nature,  as  well  as  to  those  that  are  not  foods,  and 
further,  that  they  do  not  prevent  the  organisms  from  coming  to  serious 
or  even  fatal  injury,  indicates  that  chemotaxy  depends  upon  some 
fundamental  property  of  the  protoplasm  and  is  not  a  mere  adaptation 
to  secure  special  ends,  however  well  it  may  occasionally  serve  such  a 
purpose.  In  many  cases  a  substance  which  is  attractive  at  a  low  con- 
centration proves  to  be  repellent  at  a  higher.  In  such  a  case  the  ques- 
tion arises  whether  the  repellent  action  is  due  to  the  chemical  constitu- 
tion of  the  stimulant  or  to  the  osmotic  pressure  of  its  solution.  As  the 
latter  seems  to  be  the  reason  for  the  action  in  certain  cases,  the  phe- 
nomenon is  named  osmotaxy.  It  has  not  yet  been  sufficiently  investi- 
gated, but  is  in  many  ways  parallel  to  chemotactic  irritability. 

Amount  effective.  —  The  amount  of  a  substance  which  can  act  di- 
rectively  upon  motile  organisms  is  infinitesimal.  Thus  it  was  found  that 
a  minute  capillary  into  which  the  sperms  of  a  fern  crowded,  contained,  all 
told,  less  than  three  hundred-millionths  of  a  milligram  (0.000000028  mg.) 
of  malic  acid.  Of  this,  certainly,  only  a  very  small  fraction  could  have 
reached  any  one  of  the  sperms.  Yet  relatively  the  amount  is  not  at  all 
insignificant;  for  the  estimated  weight  of  one  of  the  sperms  is  only  ten 
times  greater  than  the  total  weight  of  the  acid,  and  if  only  i/roo,ooo  of 
the  total  acted  upon  a  sperm,  the  ratio  would  be  1  :  1,000,000,  which  is 
still  10  times  the  ratio  of  a  minimum  effective  dose  of  morphin  for  the 
human  body. 

Weber's  law.  —  The  phenomena  of  chemotaxy  offer  an  excellent  illus- 
tration of  a  general  law  of  response  known  as  Weber's  law.  If  a  fern 
sperm  is  swimming  in  water,  it  will  be  diverted  toward  a  capillary  con- 
taining malic  acid  whose  concentration  is  1  part  in  100,000  of  water. 
But  if  it  is  brought  into  a  solution  too  weak  to  evoke  a  response,  say 
1  :  200,000,  it  is  so  affected  by  the  enveloping  acid  that  it  does  not  respond 
unless  the  solution  in  the  capillary  is  30  times  as  strong  as  that  by  which 
it  is  surrounded,  i.e.  30  :  200,000.  If  again  the  concentration  of  the 
acid  in  the  medium  be  raised,  say  from  1  :  200,000  to  1  :  100,000, 
the  concentration  of  the  stimulant  in  the  tube  must  be  30  times 
greater,  i.e.,  30  :  100,000,  in  order  to  evoke  response;  and  so  on.      It 


GROWTH   AND    MOVEMENT  449 

appears  from  this  that  a  sensitive  organism  becomes  adjusted  to  a  con- 
stant non-directive  stimulus,  and  then  is  unresponsive  to  an  intensity  of 
one  sided  stimulus  of  the  same  sort,  to  which  in  the  unaccustomed  state 

it  reacts.  Thus  accommodation  is  really  a  lowering  of  irritability  toward 
a  particular  stimulus.  The  noteworthy  point  is  that  it  is  a  proportional 
lowering;  for,  after  each  adjustment  has  occurred,  it  requires  a  definite 
increase  in  intensity  (in  this  particular  case  a  large  one  —  30  times  the 
constant)  to  call  forth  a  response.  Some  ratio  of  this  kind,  whether  it 
be  an  increase  of  3  times  or  30  times  the  constantly  acting  stimulus, 
ha-  been  found  to  hold  good  for  many  forms  of  response  and  in  many 
sorts  of  organisms.  In  all  cases  the  law  is  valid  for  moderate  stimuli 
only;  an  intensity  is  soon  reached  where  it  ceases  to  express  the  facts. 

The  law  was  formulated  in  1834,  with  reference  to  touch  and  sight.  It  has 
been  stated  lately  thus:  "The  smallest  change  in  the  magnitude  of  a  stimulus  whi<  h 
will  call  forth  a  response  always  bears  the  same  proportion  to  the  whole  stimulus." 

Aerotaxy.  —  One  form  of  chemotaxy  has  received  a  special  name, 
aeroiaxy,  which  signifies  that  the  air,  or  more  exactly  the  oxygen  of  the 
air,  is  the  excitant.  Certain  forms  of  bacteria  are  motile  only  when 
they  are  in  contact  with  oxygen,  and  cease  to  move  when  they  are  de- 
prived of  it.  In  so  far,  this  also  might  be  due  merely  to  respiratory 
disturbance,  just  as  many  functions  cease  when  no  oxygen  is  supplied. 
But  these  forms  also  swim  in  the  direction  from  which  the  oxygen  is 
diffusing,  and  accumulate  about  its  source.  Such  forms,  if  evenly  dis- 
tributed under  a  cover-glass,  soon  desert  the  center  and  gradually  ac(  umu- 
late  at  the  edge,  where  the  02  is  diffusing  into  the  water.  These  species, 
motile  in  oxygen,  can  be  used  as  indicators  of  photosynthesis,  because 
02  is  a  by-product. 

Ionic  stimuli.  —  All  chemotactic  reactions  to  substances  that  dissociate  in  water 
probably  rest  upon  the  specific  action  of  the  various  ions  and  molecules  present  in 
the  solution,  ami  attempts  have  l>ecn  made  to  correlate  the  action  of  the  various 
sails  and  a<  ids.  Hut  the  phenomena  are  too  complex  to  permit  satisfactory  analy- 
sis yet;  and  since  undissociable  substances  also  art  as  stimuli,  it  is  probable  that 
the  undissociated  molecules,  as  well  as  the  ions,  have  a  stimulating  action  in  many 
cases. 

Phototaxy.  —  Phototaxy  is  particularly  characteristic  of  those  organ 
isms  that  have  chlorophyll,  such  as  the  zoospores  of  algae  and  the  ciliated 

colonial  algae  like  Volvox,  Eudorina,  etc1  That  they  swim  towards 
light  of  moderate  intensity  is  not  to  be  doubted;    but   it   has  been  very 

1  Some  fungus  swarm  spurus  also  are  sensitive  to  liv;lit. 
C.   B.   &  C.    BOTANY  29 


45° 


PHYSIOLOGY 


difficult  to  determine  whether  this  response  is  due  to  the  direction  of  the 
light  rays,  or  to  the  fact  that  one  region  is  more  brightly  illuminated  than 
another.  Accumulation  certainly  occurs  in  regions  of  moderate  light, 
with  avoidance  of  the  more  shaded  or  the  more  brightly  illuminated 
portions.  The  most  exact  of  the  recent  studies  of  Volvox  shows  that 
its  orientation  is  controlled  by  the  relative  intensity  of  the  illumination 
on  different  sides  of  the  colony,  and  as  it  swims  with  a  definite  pole 
forward,  swimming  after  orientation  causes  it  to  move  nearly  parallel 
with  the  rays,  some  deflections  from  this  course  being  due  to  certain 
minor  disturbing  factors. 

In  phototaxy,  as  in  chemotaxy,  organisms  respond  both  by  orientation 
and  by  recoil,  though,  so  far  as  known,  the  latter  is  much  less  common. 
The  light  waves  vary  in  action  according  to  their  length,  the  reds  and 
yellows,  though  the  brightest,  being  quite  unstimulating,  whereas  the 
blues  are  most  effective.  Yet  this  gives  no  clew  to  the  real  nature  of  the 
excitation  or  of  the  organs  by  which  it  is  perceived. 

Geotaxy.  — Certain  organisms  have  also  been  found  to  be  geotactic.  This  prop- 
erty is  quite  distinct  from  others;  for  organisms  that  respond  alike  to  other  stimuli, 
such  as  light  and  oxygen,  may  react  differently  to  gravity,  the  one  being  positively, 
the  other  negatively  geotactic.  Upon  such  irritability  may  depend  the  al  ility  of 
the  creatures  to  rise  or  sink  through  the  water  on  occasion. 

Motion  of  cell  organs.  —  Not  unrelated  to  the  movements  of  free- 
swimming  organisms  that  have  been  described  are  the  movements  of 
organs  of  the  cell  which  take  place  within  the 
limits  set  by  the  wall.  Such,  particularly,  are 
the  movements  of  the  chloroplasts  and  the  nu- 
cleus. The  former  are  known  to  be  in  part 
responses  to  light  stimuli.  Certain  algae  of 
the  genus  Mougeotia  (Mesocarpus)  have  a 
single  platelike  chloroplast,  which  lies  in  the 
axis  of  the  cell,  facing  the  incident  light,  when 
this  is  of  appropriate  intensity.  But  if  the 
light  becomes  more  intense,  the  plate  rotates 
until  the  edge  is  presented  to  the  light.  The 
numerous  rounded  chloroplasts  of  seed  plants, 
mosses,  etc.,  alter  their  distribution  and  their 
shape  according  to  the  illumination  (figs.  68 1, 
*  682,  and  in  Part  III,  figs.  758,  759).  This 
trophe. — After  Schimper.        suggests  a  sort  of  escape  from  too  bright  light, 


Figs.  681,  682.  — Two  leaf 
cells  of  a  moss  (Atrichum 
undulatum)  seen  from  above : 
the  chloroplasts  in  68 
epistrophe;    in   682,  in   apos 


GROW  111    AND   MOVEMENT  451 

in  idea  thai  agrees  with  what  is  known  of  the  intensity  of  light  required 
for  photosynthesis  (see  p.  371)-  Yet  tne  arrangement  is  seldom  as  regu- 
lar or  complete  as  it  is  sometimes  described,  and  effective  protection 
from  light  is  secured  mainly  in  other  ways.  Aside  from  their  own 
amoeboid  movements  the  chloroplasts  are  subject  to  displacement  by 
movements  of  the  protoplast,  as  in  streaming  (below). 

The  nucleus  also  changes  its  position  in  the  cell  "  spontaneously  " 
or  in  response  to  certain  stimuli,  notably  to  wounding.  Nothing  is 
known  as  to  the  significance  or  mechanism  of  such  movements. 

Streaming.  —  In  very  many  active  cells  a  streaming  movement  of 
portions  of  the  protoplasm  has  been  observed.  The  layer  closest  to 
the  wall  does  not  participate  in  the  movement,  and  though  the  chloro- 
plasts, when  any  are  present,  are  not  necessarily  involved,  they  are  often 
swept  along  when  they  lie  deeper.  The  rate  of  the  motion  varies  with 
temperature  and  with  other  conditions  that  affect  the  general  activity  of 
the  protoplasm,  and  the  movement  may  be  entirely  stopped  by  appro- 
priate stimuli.  Nothing  is  known  as  to  the  causes  or  the  effects  of 
these  movements,  though  they  are  extremely  common  and  perhaps 
universal.  The  idea  that  they  facilitate  the  more  rapid  distribution  of 
foods  and  solutes  in  the  cells  and  so  hasten  osmotic  transfer  of  materials 
would  be  more  plausible  were  streaming  less  common  and  vigorous  in 
those  cells,  e.g.  in  hairs,  where  such  a  process  seems  of  slight  importance. 

In  some  diatoms  the  protoplasm  partly  protrudes  through  a  longitudinal  median 
slit  (the  raphe)  in  the  valves,  and  streaming  movements  in  this  outer  belt,  reacting 
against  the  water  or  the  substratum,  propel  the  cell  slowly  in  the  direction  opposite 
to  the  outer  streaming.     The  counter-stream,  of  course,  moves  within  the  cell  wall. 

Surging  movements  of  the  protoplasm  in  the  coenocytic  hyphae  of  Mucor  and 
other  fungi  have  been  seen,  but  their  causation  and  significance  are  unknown. 


6.    TURGOR    MOVEMENTS 

Motor  organs.  —  In  a  considerable  number  of  plants  thin-walled  turgid 
cells  are  so  arranged  thai  the  position  of  the  organ  of  which  they  form 
a  part  depends  upon  the  relative  turgor  of  these  cells.  In  mosl  cases  the 
organs  are  leaves,  either  foliage  or  flower  leaves,  and  the  structure  is  such 
that  the  motor  organ  curves  only  in  one  plane,  the  distal  part  rising  or 
falling  with  the  variations  of  turgor.  Examples  of  these  motor  organs 
arc  afforded  by  the  leaves  and  leaflets  of  the  Leguminosae  and  the 
Oxalidaceae,  by  the  Stamens  of  Bcrbcrls,  and  by  the  stigmas  of  Stimulus, 


45: 


PHYSIOLOGY 


they  are  also  found  in  a  considerable  number  of  families  allied  to  the 
Berberidaceae  and  Scrophulariaceae. 

Structure.  —  The  leaves  of  Leguminosae  are  usually  much  branched, 
and  the  primary  motor  organ,  when  present,  is  located  at  the  base  of 
the  main  petiole.  In  many  cases  there  are  also  motor  organs  (secondary) 
at  the  origin  of  the  secondary  petioles,  and  if  the  leaf  is  ternately  com- 
pound the  petiolules  or  stalks  of  the  leaflets  are  motor  organs.     Thus 

Mimosa  has  primary,  second- 
ary, and  tertiary  motor  organs 
(fig.  683) ;  but  the  red  and  sweet 
clovers  have  only  one  set,  the 
stalks  of  the  leaflets.  The 
motor  organ  consists  of  all  or 
a  portion  of  the  petiole  or  peti- 
olule,  modified  by  changes  in 
the  position  of  the  vascular 
bundles,  and  by  an  excessive 
development  of  the  paren- 
chyma of  the  cortex.  Through 
the  greater  part  of  the  leaf 
stalk  the  vascular  bundles  lie 
at  some  distance  from  the 
center,  surrounding  a  distinct 
pith,  and  within  a  cortex  of 
moderate  thickness.  In  the 
motor  organ,  however,  they  ap- 

Leaf  of  Mimosa  in  open  and  closed  proach  one  another  so  closely 
positions.  —  From  Part  III.  ,,     ,   ,,  .  , 

that  there  is  scarcely  any  cen- 
tral pith,  and  they  form  a  shaft,  elliptical  or  kidney-shaped  in  section. 
Outside,  the  cortex  is  correspondingly  larger,  and  its  cells  are  usually 
somewhat  different  from  the  rest.  As  a  whole  the  motor  organ  is  some- 
times thicker  than  the  other  part  of  the  petiole,  but  it  is  quite  as  likely 
to  be  smaller ;  in  all  cases,  however,  the  relative  increase  of  the  cortex 
in  cross  section  gives  the  impression  of  a  cushion  of  parenchyma.1 
In  this  region  the  cells  are  rather  regular  in  form,  approximately 
cylindric,  and  with  smaller  intercellular  spaces  than  in  the  nutritive 
regions.  Intercellular  spaces  are  present,  however,  at  the  junction  of 
three  or  more  cells. 

1  This  is  the  reason  for  a  technical  name  applied  to  the  motor  organ,  the  pulvinus. 


Fig.  683. 


GROWTH     WD    MOVEMEN  V 


453 


Mechanism.  —  It  is  evident  that  the  central  position  of  the  vascular 

bundles  permits  flexure  more  readily  than  if  they  wire  Mattered  and 
more  peripheral;  while  the  peripheral  position  <>f  the  thin-walled  cells  of 
the  cortex  is  such  that  any  variation  in  their  turgor  will  produce  a  cur- 
vature, the  side  with  less  turgor  becoming  concave,  since  its  cell-  no 
longer  oppose  fully  the  turgid  cells  of  the  opposite  side.  Correspond 
ingly,  the  parts  beyond  the  curving  motor  organ  will  be  displaced  by  it. 
These  turgor  variations,  due  to  modified  permeability,  being  usually 
restricted  to  the  upper  and  lower  sides  of  the  motor  organ,  the  distal 
parts  arc  moved  up  and  down.  Since  the  relaxed  cells  may  recover  tur 
gidity  and  the  turgid  cells  become  flaccid,  the  notable  feature  of  all  such 
movements  is  that  the  changes  in  the  cells  are  reversible;  whereas  the 
cell  changes  involved  in  growth  are   irreversible  (or  soon  become  so). 

The  motor  organs  of  stigmas  and  stamens  are  essentially  similar  to  those  of 
foliage  leaves,  but  simpler,  since  vascular  tissues  are  slightly  or  not  at  all  developed, 
and  almost  the  whole  tissue  is  parenchymatous. 


Autonomic  movements.  —  The  variations  in  turgor  are  sometimes 
autonomic,  that  is,  determined  by  causes  unknown  and  apparently  in- 
ternal to  the  plant,  but  more  commonly  they 
are  controlled  by  external  stimuli.  Autonomic 
movements  are  not  at  all  uncommon,  but 
they  are  mostly  too  slow  to  be  observed  easily 
without  apparatus,  and,  when  sought,  are 
often  masked  by  more  obvious  movements 
(see  p.  457).  The  classical  and  almost  the 
only  striking  example  of  easily  seen  move- 
ments is  offered  by  Desmodium  gyrans,  whose 
lateral  leaflets  (fig.  684)  are  constantly  rising 
and  falling  under  favorable  conditions.  These 
movements,  sometimes  uniform,  but  usually 
jerky,  are  not  very  rapid,  for  a  complete  up- 
and-down  movement  requires  2-4  minutes. 
The  fall  is  more  rapid  than  the  rise  (for  ex- 
ample, 45  sec.  as  against  70);  and  as  the  tur- 
gor variations  tend  to  fluctuate  regularly  to 
right  and  left  of  the  vertical  plane,  the  tip  of 
each  leaflet  describes  a  narrow  ellipse.  The  reason  for  these  move- 
ment- is  unknown,  nor  are  they  known  to  be  of  any  value  to  the  plant. 


Fie.  684.  —  Leaf  of  tele- 
graph    plant     (Desmodium 

gyrans),  natural  si/.o:  /,  /, 
lateral  leaflets  which  show 
autonomous  movements;  the 
terminal  lea  fie t  in  the 
depressed     position.  —  After 

I'l  1   111   K. 


454 


PHYSIOLOGY 


Under  unfavorable  conditions  they  cease,  but  the  plant  may  still  be 
able  to  respond  to  external  stimuli  like  others  about  to  be  described. 

Paratonic  movements.  —  The  terminal  leaflet  of  Desmodium  gyrans, 
like  leaves  of  other  members  of  the  'bean  family,  exhibits  paratonic 
movements  {i.e.  those  due  to  special  stimuli,  not  tonic;  opposed  to  auto- 
nomic). Moreover,  some  plants  whose  leaflets  ordinarily  exhibit  only 
paratonic  movements,  may  make  autonomic  ones  under  exceptionally 
favorable  conditions.  Thus  it  would  seem  that  there  is  no  fundamental 
difference  in  the  two,  and  when  the  precise  stimuli  that  initiate  the  move- 
ment are  discovered,  autonomic  movements  may  all  be  relegated  to  the 
paratonic  category. 

Turgor  movements  due  to  external  stimuli  are  numerous  and  easily 
observed,  but  except  in  a  few  striking  cases  they  are  not  rapid  enough 
to  be  seen  by  watching  for  a  brief  time.  The  stimuli  initiating  the  move- 
ments are  of  the  most  varied  character;  contact,  gravity,  and  changes 
of  light  and  temperature  being  the  most  common. 

Contact  movements.  —  If  the  stamens  of  the  barberry  (Berberis)  be 
touched  near  the  base  at  the  time  when  they  are  shedding  pollen,  they 
suddenly  fly  up  and  inward,  carrying  the 
anthers  close  to  the  stigma.  After  a  short 
time  they  resume  their  former  position  against 
the  petals.  The  filaments  of  the  Cynareae, 
a  tribe  of  Compositae,  shorten  instantly  on 
being  touched  (the  reaction  time  is  less  than 
i  sec),  dragging  the  coherent  anthers  quickly 
down  over  the  style,  whose  hairs  scrape  out 
the  pollen  like  a  pipe  cleaner.  In  Centaurea 
americana,  this  contraction  continues  for 
7-13  seconds,  and  after  a  minute  the  rest 
position  is  again  reached. 

Probably  the  best  known  of  the  rapid 
contact  movements  are  those  of  the  species 
of  Mimosa  and  Biophytum,  the  "  sensitive 
plants."  In  Mimosa  the  leaflets  are  carried 
by  the  motor  organs  forward  and  upward 
until  the  upper  faces  are  pressed  together, 
while  the  primary  motor  organ  drops  the 
whole  leaf  (fig.  683,  p.  452).  Another  famous  example  is  the  quick 
closure  of  the  "  fly-trap  "  of  Dionaea  (figs.  657,  658,  p.  386).     Here 


Fig.  685.  —  Leaf  of  sundew 
(Drosera  rotundifol ia)  with  half 
of  the  tentacles  inflexed  from 
stimulation.  —  Adapted  from 
Keener. 


CROW  I'll    AND   MOVEMENT  455 

tlu-  motor  organ  lies  along  the  central  rib,  between  the  two  lobes  of  the 
leaf,  and  when  an  insect  tone  lies  one  of  the  three  sensitive  bristles  on 
either  face,  these  lobes  shut  together  quickly  like  the  jaws  of  a  trap, 
and  their  interlocking  teeth  prevent  the  prey  from  crawling  out  easily. 
After  a  time  the  superficial  glands  pour  out  a  secretion  containing  an 
enzyme  that  digests  the  proteins,  and  these  are  absorbed  and  utilized 
as  food.  After  several  days  the  trap  again  opens.  Somewhat  slower 
movements  are  made  by  the  "  tentacles  "  of  Drosera  (fig.  685).  When 
an  insect  is  entangled  in  the  viscid  secretion  at  the  tips  of  these  leaf  lobes, 
its  struggles  furnish  a  stimulus  which  results  in  the  incurving  of  all,  until 
it  is  completely  enveloped  in  their  secretion,  which  then  changes  char- 
acter, bediming  digestive,  and  so  prepares  the  proteins  for  absoq)tion 
(see  p.  388). 

Gravity  movements.  —  Gravity  cannot  act  as  a  stimulus  unless  the 
plant  be  displaced.  If  a  potted  bean  plant  be  turned  upside  down  or 
laid  on  the  side,  in  a  few  hours  the  motor  organs  become  curved  so 
as  to  bring  the  leaves  again  into  the  usual  position,  or  as  near  to  it  as 
possible. 

Photeolic  movements.  —  The  most  striking  movements  are  the  regular 
ones  produced  by  motor  organs  under  periodic  stimulation  by  variations 
in  the  intensity  of  light  (and  temperature).  These  have  been  known 
under  the  misleading  name  of  "  sleep  movements,"  because  they  are 
notable  at  nightfall.  However,  they  have  no  similarity  whatever  to  tin- 
relaxed  position  assumed  by  animals  in  sleep,  nor  do  they  bring  any 
recovery  from  fatigue.  On  the  contrary,  the  nocturnal  position  is  one  of 
precisely  as  much  strain  as  the  diurnal  one,  since  the  resistance  of  the 
motor  organ  to  bending  is  measurably  the  same;  and  even  the  position 
is  as  likely  to  be  erect  as  drooping. 

Technically  they  have  been  called  nyctitropic  movements,  but  as  the  curvature 
is  not  a  tropic  one  this  term  is  objectionable, and  the  more  so  as  the  movements  are 
quite  as  much  associated  with  day  as  with  night.  They  are  best  called  photeolic 
i/r.  light  variation)  movements,  because  the  illumination  is  chiefly  responsible  for 
them,  though  corresponding  fluctuations  in  temperature  accompany  the  changes 
in  light  and  sometimes  cooperate  in  selling  up  the  movement. 

Photeolic  movements  consist  of  a  rising  or  falling,  a  forward  or  back- 
ward movement,  of  the  entire  leaf  and  (if  the  leaf  be  compound)  of  all 
the  leaflets  as  well;  or  the  leaflets  alone  of  a  compound  leaf  may  exhibit 
such  movements.  The  change  in  the  leaves  of  the  common  purslane 
(figs.  686,  687)  will  make  clear  the  general  <  haracter  of  these  changes 


456 


PHYSIOLOGY 


of  position,  which  are  executed  by  differences  of  turgor  on  opposite 
sides  of  motor  organs  appropriately  situated.  Inasmuch  as  the  changes 
in  illumination  are  not  sudden  (in  nature),  it  should  be  expected  that 
the  movements  would  not  be  restricted  to  morning  and  nightfall.  In 
fact  it  can  be  shown  that  there  is  really  a  slow  variation,  so  that  in  the 
brightest  hours  of  the  day  the  blades  reach  their  highest  or  lowest  posi- 
tion, the  opposite  being  attained  in  the  maximum    darkness.      As    the 

changes  in  the  inten- 
sity of  the  light  are 
most  marked  at  dawn 
and  at  dusk,  the 
changes  of  position 
are  then  most  rapid 
and  so  attract  atten- 
tion. 

Persistence.  —  To 
these  periodic  vari- 
ations in  light  the 
plant  becomes  habit- 
uated, and  even  if 
they  are  not  allowed 
to  occur,  as  when  a 
plant  is  kept  in  con- 
tinuous  darkness  or 

Figs.  686,  687. —  Shoot  of  the  purslane  {Portulaca  olcra-  .  ,.    ,         , 

cea),  photographed  from  identical  position  at  2  p.m.  (686)  continuous  llgnt,  tne 
and  at  8.30  p.m.  (687);  note  that  the  older  leaves  show  little  movements  continue, 
change  of  position.  -  From  photograph  by  Land.  ^  diminishing  am_ 

plitude,  for  a  considerable  time  (3-5  days)  before  they  cease  entirely. 
The  normal  periodic  stimulation  seems  to  have  impressed  upon  the 
protoplast  a  rhythmic  variation  in  turgor,  so  that  it  cannot  at  once 
cease  the  customary  action  even  when  no  stimulus  demands  a  reaction 
(fig.  688). 


When  these  movements  are  ceasing,  there  come  to  view  similar  ones  which  are 
usually  masked  by  the  photeolic  reactions.  These,  however,  are  autonomous ; 
they  are  much  less  extensive  and  have  a  much  shorter  period  than  the  others. 
When  sought,  they  can  be  observed  even  in  the  presence  of  the  photeolic  movements. 
They  consist  of  a  pendulum-like  swinging  of  the  leaf  or  leaflets,  up  and  down  (some- 
what as  in  Desmodium,  fig.  684;  see  also  fig.  689),  whose  advantage  and  effects 
are  alike  obscure. 


GROWTH    AND    M<  >VEMENT 


457 


Advantage. — The  benefits  of  photeolic  movements  have  been  vari 
ously  imagined.  They  have  been  supposed  to  prevent  injury  to  the 
leaves  by  frost,  since  the  folded  position  diminishes  radiation;  or  to 
prevent  the  formation  of  dew,  so  that  transpiration  may  begin  promptly 
in  the  morning.  The  difficulty  with  the  firsl  of  these  ideas  is  thai  frost 
does  not  occur  in  the  regions  where  Leguminosae,  which  exhibit  them 
more  strikingly  than  any  other  family,  most  abound;  furthermore,  a 
temperature  approaching  o°  C.  would  render  response  impossible.  The 
second  explanation  involves  the  assumption  that  transpiration  is  a  valu- 


FiGS.  688,689. — Autographic  rec- 
ords of  leaf  movements:  dates  and 
hours  of  the  day  are  given  below;  12 
noon,  24  midnight;  the  horizontal 
median  line  represents  the  line  the 
recording  point  would  have  described 
had  the  leaf  remained  quiet,  moving 
neither  toward  the  diurnal  (day)  nor 
tin  nocturnal  (i.igki)  positions;  the 
bla<  k  strips  mark  the  periods  of  dark- 
ening, which  have  no  relation  to  the 
natural  alternation  of  day  ami  night; 
688,  photeolic  movements  of  leaf  of 
Albizzia  lophantha;  after  a  period  of 
constant  illumination  the  plant  was  subjected  to  l>  hi.  period-  of  alternating  darkness 
and  light,  then  to  continuour  light,  and  finally  to  3  hr  periods  of  alternate  darkness 
and  light;  note  the  persistence  in  light  t< ).  t.  25-27)  of  the  movements,  which  gradually 
disappear;  6S0,  photeolic  and  autonomous  movements  of  leaf  of  Phaseolus  vitcllinus, 
the  latter  restricted  to  the  reversed  periods  (lf  illumination  (<>  P.M.  to  6  A.M.);  note  the 
l.ii'  of  the  response  in  the  former.  —  After  PFEFFER. 

able  function  which  the  plant  promote?,  instead  of  a  danger  that 
menaces  its  very  life.  It  is  difficult  to  conceive  the  significance  of  these 
movements  in  terms  of  welfare,  and  it  is  quite  possible  that  they  have 

tlotie. 

Other  stimuli.  -  Changes  in  temperature,  which  often  coincide  and  cooperate 
with  changes  of  light  in  producing  photeolic  movements,  may  acl  alone,  and,  when 

Sufficient,    call    forth    like    responses.      Severe    injury,    even  when  wrought  without 

mechanical  disturbance,  as  by  burning  with  a  lens,  will  also  stimulate  the  motor 
organs  to  curvature.     So  will  a  variety  of  other  stimuli. 


458  PHYSIOLOGY 

Growth  movements  and  turgor  movements.  — The  intimate  relations  that  exist 
between  turgor  and  growth,  as  well  as  the  suddenness  of  their  response  under  favor- 
able conditions,  make  it  possible  that  the  first  curvature  of  tendrils  (see  p.  471)  is 
due  to  quick  alterations  in  the  turgor  of  the  cortical  parenchyma.  If  this  be  true, 
the  turgor  curvature  is  followed  promptly  by  unequal  growth,  to  which  irreversible 
process  the  more  permanent  and  more  important  of  the  tendril  movements  are  due. 
Their  behavior  will  therefore  be  discussed  in  connection  with  growth  movements. 
Indeed  it  is  not  improbable  that  turgor  changes  underlie  all  such  movements,  though 
they  are  not  apparent. 

In  many  plants  whose  leaves  have  no  well-defined  motor  organs  there  exist  slight 
modifications  of  structure  looking  in  the  same  direction,  with  movements  of  less 
extent  than  those  executed  by  well-developed  motor  organs.  Moreover,  there  are 
to  be  found  similar  movements  in  young  leaves  that  have  no  trace  of  motor  organs, 
but  these  movements  cease  by  the  time  the  leaves  have  attained  mature  size. 
(Compare  young  and  old  leaves  in  figs.  686,  687.)  Doubtless  growth,  that  is,  irre- 
versible changes  in  the  size  of  the  cells,  as  contrasted  with  the  reversible  changes 
produced  by  turgor,  cause  these  movements. 

From  the  foregoing  it  is  evident  that  no  hard  and  fast  line  can  be  drawn 
between  the  displacements  due  to  turgor  and  those  due  to  growth.  In  fact  there  are 
all  gradations  between  them.  Therefore,  the  separate  treatment  must  not  be  per- 
mitted to  establish  in  the  mind  too  sharp  distinctions  ;  for  distinctions  are  valuable 
chiefly  as  conveniences  to  the  memory  ;  they  have  usually  slight  basis  in  nature. 

7.    TROPISMS 

Growth  curvatures.  —  It  is  a  matter  of  common  observation  that  the 
various  parts  of  a  plant  have  definite  positions.  If  they  are  mechani- 
cally displaced,  the  usual  position  often  is  resumed  after  a  time  by  cur- 
vature. Again,  if  some  external  force  acts  upon  them  from  an  unac- 
customed direction,  a  curvature  may  result,  restoring  the  customary 
relations  so  far  as  may  be.  Some  of  these  curvatures  have  been  con- 
sidered; namely,  those  that  are  due  to  changes  of  turgor.  But  a  much 
larger  number  are  due  to  growth,  because  few  plants  have  such  a  struc- 
ture as  to  permit  turgor  variations  to  move  an  organ.  On  the  contrary, 
every  plant  has  some  part  where  growth  is  either  in  progress  or  can  be 
initiated,  and  consequently  a  curvature  can  be  induced,  if  by  appropriate 
mechanisms  the  amount  or  rate  of  growth  can  be  modified  locally. 
Practically  all  plants  have  such  mechanisms,  which  are  set  into  operation 
by  various  external  stimuli.  It  will  be  most  convenient  to  consider  these 
tropic  curvatures  according  to  the  stimulus  that  induces  the  reaction. 

Parallelotropic  and  plagiotropic  organs.  —  Observation  shows  that  in 
certain  plants  the  main  axes  respond  to  a  tropic  stimulus  by  placing  them- 
selves parallel  to  the  direction  from  which  the  stimulus  acts,  while  other 


GROWTH    AND    MOVEMENT  459 

parts,  such  as  the  branches  or  leaves,  set  themselves  at  a  definite  angle  to 
the  line  of  the  stimulus.  Other  plain-  may  place  even  the  main  axes 
at  an  angle  to  the  stimulus.  This  difference  of  behavior  i-  expressed  by 
the  terms  parallelotropic  and  plagiotropic,  applied  to  the  organ  concerned. 

Because  responses  to  tropic  stimuli  lead  so  often  to  the  erect  position  oi  ax<  , 
such  axes  were  first  called  orthotopic  organs,  and  their  correlates  were  called 
plagiotropic,  with  reference  merely  to  position.  No  confusion  can  arise  from  the 
substitution  of  the  more  specific  term  parallelotropic,  and  the  use  of  plagiotropic  in 
a  somewhat  modified  sense. 

(1)   Geotropism 

The  stimulus.  —  No  force  acts  so  constantly  and  so  equally  in  all  parts 
of  the  earth  and  in  all  situations  as  gravity.  It  might  he  expected,  there- 
fore, that  it  would  have  some  influence  upon  the  position  that  the  parts 
of  plants  assume.  If  then-  were  nothing  more  to  be  observed  than  that 
the  main  stems  of  so  many  plants  in  all  countries  are  directed  away 
from  the  center  of  the  earth,  this  would  suggest  the  agency  of  some 
general  stimulus.  But  it  is  easy  to  observe  that  as  soon  as  a  plant  stem 
which  usually  grows  erect  is  overthrown,  curvatures  occur  in  the  younger 
parts  that  again  direct  the  apex  upward,  though  the  older  parts  are 
unable  to  erect  themselves.  Fallen  trees,  and  corn  or  other  cereals 
beaten  down  by  wind  and  rain,  offer  many  examples,  and  the  simplest 
experiments  suffice  to  demonstrate  the  main  facts;  namely,  that  gravity 
is  the  stimulus,  and  unequal  growth  the  end  reaction. 

The  first  demonstrative  experiments  were  conducted  at  the  beginning 
of  the  last  century,  by  affixing  boxes  to  the  rim  of  a  wheel,  which  could 
be  rotated  either  in  the  vertical  or  the  horizontal  plane,  and  planting 
seeds  in  these  boxes.  When  the  seedlings  appeared  on  the  vertically 
placed  wheel,  they  seemed  to  have  quite  lost  their  way,  growing  in  any 
direction  in  which  they  happened  to  be  pointed  when  they  broke  through 
the  soil;  and  some  did  not  even  emerge.  On  the  horizontal  wheel, 
however,  no  difference  was  apparent  when  it  was  rotated  slowly;  but 
when  it  was  turned  rapidly  enough  to  introduce  a  considerable  centrifu- 
gal acceleration  ("  centrifugal  force  "),  the  usual  position  of  the  axes 
was  changed,  the  stems  which  would  normally  grow  erect  tending  to 
direct  themselves  toward  the  center  of  the  wheel,  and  the  primary  root-. 
which  usually  grow  downwards,  glowing  toward  the  periphery  ;  and  these 
tendencies  were  the  more  pronounced  the  more  rapid  the  rotation. 

This  mode  of  experimentation  is  universally  used  when  one  wishes  to  equalize 
or  modify  the  u>  tion  of  any  one-sided  stimulus.      In  all  such  experiments  it  is  essen 


460  PHYSIOLOGY 

tial  to  arrange  the  plants  so  that  the  only  factor  in  their  environment  that  is  altered 
is  the  direction  from  which  the  stimulus  acts.  The  clumsy  wheel  has  been  replaced 
by  the  modern  cliiwstat,  a  disk  to  which  potted  plants  can  be  conveniently  attached 
and  capable  of  rotation  in  any  plane,  continuously  or  intermittently,  at  a  very  even 
speed  •  by  strong  clockwork  or  by  a  water  or  electric  motor.  The  centrifuge  is  a  modi- 
fication whose  disk  is  driven  at  a  high  speed  when  centrifugal  acceleration  is  to 
be  compared  with  gravitational. 

Parallelotropic  organs.  —  The  behavior  of  parallelotropic  and  plagio- 
tropic  organs  differs  in  certain  particulars.  The  former  will  first  be 
considered.  Parallelotropic  stems  in  responding  to  gravity  curve  so 
as  to  erect  their  apices  when  displaced.  Primary  roots,  which  are  usu- 
ally directed  straight  downwards,  when  displaced  respond  by  turning  the 
tip  toward  the  earth.  These  responses,  in  quite  opposite  directions,  arise 
from  an  identical  original  stimulus.  By  some  mechanism  within  the 
plant  body  the  end  reaction  is  made  different.  It  is  convenient  to  dis- 
tinguish the  difference  by  assuming  some  difference  in  the  sensitiveness. 
So  the  special  term  positive  geotropism  or  progeotropism  is  used  to  desig- 
nate the  property  by  which  the  growing  point  is  directed  toward  the 
center  of  the  earth,  and  negative  geotropism  or  a po geotropism  that  by 
which  the  tip  is  turned  away  from  it.  The  curvature  might  be  due  (a) 
to  unequal  retardation  of  growth  along  both  sides;  or  (b)  to  unequal 
acceleration  of  growth  along  both  sides;  or  (c)  to  an  unchanged  rate  of 
growth  on  one  side  with  either  acceleration  or  retardation  on  the  other  ; 
or  finally  (d)  to  simultaneous  retardation  on  one  side  and  acceleration 
on  the  other.  It  has  been  determined  that  usually,  in  both  stems  and 
roots,  gravity  accelerates  growth,  but  the  segments  are  unequally  affected 
according  to  position  (case  b).  In  the  one  case  (apogeotropism),  the 
lower  side  is  caused  to  grow  more  rapidly  than  the  upper  ;  in  the  other 
(progeotropism),  the  upper  side  grows  more  rapidly  than  the  lower. 
How  this  difference  in  action  is  brought  about  is  quite  unknown. 

Course  of  curvature.  —  The  course  of  curvature  in  a  parallelotropic 
stem  continuously  stimulated  by  being  laid  horizontal  shows  an  interest- 
ing example  of  "  after-effects."  The  reaction  time  is  usually  some  hours 
in  length.  When  the  apex  has  reached  the  erect  posture  again,  it  might 
be  supposed  that  it  would  go  no  further.  On  the  contrary,  it  is  carried 
past  the  vertical,  responding  to  the  excitation  set  up  some  hours  before. 
Being  thus  carried  beyond  the  position  of  equilibrium,  it  is  stimulated 

1  Otherwise  any  exact  experiments  may  be  vitiated  by  errors  due  to  unequal  stimula- 
tion, a  common  fault  with  makeshift  clock  clinostats,  which  suffice,  however,  for  elemen 
tary  demonstrations. 


GR<  'Will    AND    M<  <\  i  Mi  N  T 


461 


to  a  reverse  curvature,  ami  tliis  also,  by  rea  on  of  continued  stimulation 
during  the  long  reaction  time,  may  again  carry  the  tip  past  tin-  vertical; 

thus,  only  by  a  series 
of  pendulum-like 

swings  is  tlu'  position 
of  equilibrium  at- 
tained. The  succes- 
sive positions  of  the 
stem  of  Impatiens 
shows  the  way  in  which 
such  a  stem  erects  it- 
self (fig.  690).  It  shows 
also  that  the  curvature 
begins  in  the  region 
of  most  active  growth 
and   gradually  affe<  ts 


1  h..  6()o.  —  Successive  positions,  from  photographs,  <>f 
Impatiens  glanduligera  in  erecting  itself  from  tin-  horizon- 
tal.—  After  PFEFFER. 


less  active  regions,  becoming  permanent   finally  as  the  tissues  of   the 
growing  region  most  remote  from  the  apex  cease  to  grow. 

That  the  curvature  appears  in  the  region  of  most  active 
elongation  is  clearly  shown  by  the  behavior  of  certain  roots. 

If  a  suitable  one  be  marked  at  intervals  of   1  mm.  ami  then 

fixed  in  a  horizontal  position,  it  will  be  found  aftersome  hours 

^-*- — ",_~~'  that  curvature  is  taking  place  in  the  third  and  fourth  of  these 

f?C  692  divisions;    after  twenty-four  hours  it  is  easy  to  see  that  the 

i/  V.  sci  ond  and  third  divisions  have  grown  most,  though  the  c  hief 

urvature  still  persists  in  the  fourth  division  that  was  grow- 

ing  most  rapidly  (figs.  691-603). 


Presentation  time.  —  It  is  not  necessary  to  con- 
tinue stimulation  until  the  reaction  appears.  In 
other  words  reaction  time  is  longer  than  presenta- 
tion time.  These  periods  are,  of  course',  very  sari- 
able.  The  shortest  presentation  time  recorded  for 
geotropic  curvature  is  2-3  minutes  (cut  shoots  of 
Capsella,  hypocotyls  of  Helianthus,  and  peduncles 
placed    horizon-    <>f  PlatUago).      In   many  plants  it  is  15    25  minutes; 

t.d;   69a,  seven  hours    jn   |t._   sensitive  plants  it   is  double  or  treble  this,  or 
later  ;      6,,;,,     twenty-  ' 

three  hours  later.—    even   extends   to  several    hours.     Moth   periods  are 
After  S.miis.  greatly  inlhicnced  by  temperature.      Thus,  a  seedling 

of  Vicia  Faba,  having  at  140  C.  a  presentation  time  of  70  minute-  and  a 
reaction  time  of  120  minutes,  hail  these  periods  at  300  C.  respectively 


Figs.    601-60 3. — 
( reol  ropii  1  urvatureof 
a  root  of  Vit  iii  Faba 
60 


4(>2  PHYSIOLOGY 

10  minutes  and  48  minutes.  The  longer  the  stimulation,  other  things 
being  equal,  the  more  marked  the  curvature  ;  from  which  it  is  evident 
that  there  is  an  increase  of  the  excitation  with  continued  stimulation, 
and  thereby  the  end  reaction  becomes  more  marked. 

Summation.  —  Contrariwise,  it  should  be  expected  that  stimulation 
too  short  to  result  in  curvature  would  not  be  without  effect.  That  it 
does  produce  excitation  is  shown  by  the  fact  that  if  a  plant  be  placed  alter- 
nately horizontal  and  erect,  each  period  of  stimulation  being  shorter 
than  the  presentation  time  for  that  particular  plant,  and  the  interval  of 
rest  shorter  than  is  needed  for  recovery,  curvature  will  finally  occur. 
Evidently  this  is  a  cumulative  effect;  yet  it  is  not  a  summation  of  the 
total  successive  excitations  that  occur  during  the  times  of  horizontality, 
a  j-  but  only  of  the  re- 

sidual excitation. 
For,  if  a  suitable 
plant  be  placed 
horizontal  for  30 
minutes  continu- 
ously, the  reaction 
curvature  is  more 
C  pronounced    than 

Diagram:  for  explanation,  see  text.  .?   ..    ,  1        j 

5  ^  if  it  be  so  placed 

for  ten  3-minute  periods  at  10-minute  intervals.  Clearly,  while  erect, 
the  preceding  excitation  is  slowly  disappearing,  and  if  the  interval  before 
the  next  stimulation  is  too  long,  recovery  will  be  complete  and  no 
evidence  of  the  excitation  will  appear  in  the  form  of  curvature.1  In 
such  experiments,  therefore,  it  is  necessary  to  apportion  properly  the 
intervals  of  rest  and  stimulation. 

Rotation.  —  From  the  above  considerations  it  will  be  evident  that  when 
a  plant  is  rotated  in  the  horizontal  plane  on  a  clinostat,  its  failure  to  exe- 
cute any  curvature  is  not  at  all  due  to  a  lack  of  excitation,  for  while  the 
side  a  of  the  stem  is  passing  through  quadrant  A  of  its  rotation  (fig.  694), 
quadrants  a  and  c  are  under  stimulation  almost  as  though  for  a  corre- 
sponding time  the  stem  were  at  rest.  But  these  sides  remain  under  stimu- 
lation for  less  than  the  presentation  time  and  so  the  excitation  does  not 
suffice  for  the  end  reaction.     When  side  a  has  passed  into  quadrant  C 

1  It  has  been  suggested  that  during  the  periods  of  no  stimulation  a  counter-excitation  is 
set  up;  but  simple  recovery  from  excitation  seems  sufficient  to  account  for  all  the  facts 
known.     The  process  is  apparently  analogous  to  recovery  from  fatigue. 


GROWTH    AND    MOVEMENT 

of  its  rotation  an<l  c  into  .1,  any  residual  excitation  from  the  former  posi 
tion  is  balanced  by  excitation  that  would  lead  to  a  contrary  reaction. 
All  the  while,  therefore,  the  plant  is  under  ex<  itation,  whi<  b  i>  the  greater 
the  more  opportunity  there  is  for  summation;  and  if  the  responses  win- 
not  contrary  the  one  to  the  other,  curvature  would  show  itself.  The 
net  result  upon  the  rotated  plant  is  that  growth  is  at  first  accelerated  as 
compared  with  a  control  plant  rotated  in  the  vertical  plane;  but  long- 
continued  rotation  leads  to  fatigue  and  no  response. 

Position  of  equilibrium.—  In  order  that  a  parallelotropic  axis  be  in 
a  position  of  stable  equilibrium  with  respect  to  gravity,  it  must  not  only 
be  parallel  to  its  direction,  but  the  stem  must  be  erect  and  the  root  pointed 
downward.  There  is  a  polarity  which  must  be  conserved.  Though 
the  strictly  inverted  position  for  either  roots  or  stems  is  one  of  little 
stimulation  or  possibly  of  none,  it  is  a  position  of  such  instability  that 
the  slightest  deviation  leads  to  stimulation,  which  results  in  decided  <  ur- 
vatures  and  recovery  of  the  normal  position.  Much  study  has  been 
given  also  to  the  position  of  maximum  stimulation.  The  general  results 
are  most  strongly  in  favor  of  a  oo°  deviation  from  the  normal,  as  agai-Hst 
1 3 5°  or  any  intermediate  angle. 

Variable  intensity.  —  By  comparing  centrifugal  acceleration  with 
that  due  to  gravity,  it  has  been  shown  that  it  produces  the  same  curva- 
tures. So  while  it  is  not  possible  to  alter  appreciably  the  intensity  of 
gravity,  it  is  possible  to  vary  this  corresponding  stimulus.  Experiments 
in  this  line  show  that  as  the  centrifugal  acceleration  is  increased  or  di- 
minished, the  reaction  time  is  shortened  or  lengthened,  but  whether  pro- 
portionately or  not  is  uncertain. 

Thus,  in  earlier  experiments  with  a  root  of  Vici<i,  whose  usual  reaction  time  at 
i  g  '  was  90-100  min.,  when  the  centri-acceleration  was  equal  to  35—38^,  the  reai  lion 
time  was  scarcely  more  than  halved  (45  min.);  and  when  it  was  reduced  to  0.001  g,  the 
reaction  time  was  barely  quadrupled  (6  hr.).  In  some  late  experiments,  however, 
a  root  of  Vicia,  which  reacted  in  8  min.  at  1  g,  reacted  in  0.25  min.  with  27 g.  Here 
the  ratio  is  32 :  27,  a  change  in  reaction  time  nearly  proportionate  to  the  change  in 
stimulus. 

Perceptive  region.  —  It  is  extremely  difficult  to  locate  beyond  question 
the  exact  region  where  the  geotropic  stimulus  is  perceived.  In  stems 
perception  does  not  seem  to  be  localized.  If  the  statolith  theory  of  geo- 
perception  be  true,  it  takes  place  probably  in  the  starch  sheath,  a  layer 
of  cells  around  the  vas<  ular  1  ylinder. 

1  1  g  =  the  normal  acceleration  due  t<>  gravity, 


464 


PHYSIOLOGY 


The  most  thorough  experiments,  however,  have  been  made  upon  roots, 
and  these  seem  to  show  that  perception  takes  place  mainly  in  the  very  tip, 
within  a  zone  little  more  than  a  millimeter  long,  including  the  root  cap. 
Indeed,  the  inner  portions  of  the  root  cap  itself  are  believed  to  be  the 
cells  most  concerned.  But  the  results  also  show  that  the  growing  region 
has  some  perceptivity. 

This  conclusion  rests  upon  evidence  derived  mainly  from  three  modes  of  experi- 
mentation: (a)  Decapitation.  Cutting  off  the  terminal  millimeter  or  two  leaves 
the  root  still  capable  of  weak  response,  after  recovery  from  the  shock,  (b)  Me- 
chanical deformation.  If  root  tips  are  made  to  grow  into  glass  slippers  (figs.  695, 
696),  or  against  a  glass  plate,  so  that  the  terminal  millimeter  is  bent  at  right  angles  to 


Figs.  695,  696.  —  Roots  of  Vicia  Faba  with  tips  in  glass  slippers  :  695,  a,  b,  c,  three 
stages  in  the  curvature  of  the  same  root,  o  to  20  hours  ;  696,  a,  b,  two  stages  of  the  same 
root ;  b,  1 8  hours  after  being  placed  in  position  a.  —  After  Czapek. 

the  body  of  the  root  and  therefore  can  be  placed  in  the  position  of  stimulation  while 
the  rest  is  not  (or  vice  versa),  responses  show  the  dominance  of  the  excitation  at  the 
tip  over  that  in  the  growing  region ;  but  the  conclusion  that  the  tip  alone  is  per- 
ceptive is  not  warranted,  (c)  Rotation.  Experiments  in  which  the  roots  are  fixed 
on  a  centrifuge,  deviating  1350  from  their  normal  position,  permit  responses  to  be 
varied  at  will,  according  to  the  extent  of  the  root  tip  beyond  the  axis  of  rotation. 
In  all  cases,  if  the  stimulus  to  the  growing  region  is  to  determine  the  response,  it 
must  be  several  times  greater  than  is  needed  at  the  tip.  Anatomical  facts,  in  con- 
nection with  the  statolith  theory  of  geoperception,  support  the  physiological  evidence 
above  cited  (fig.  697). 

Statolith  theory.  —  In  its  original  form  this  theory  was  purely  specu- 
lative. It  postulated  in  the  protoplasts  of  perceptive  cells  minute  vacu- 
oles, beyond  the  limits  of  microscopic  vision,  filled  with  a  fluid  in  which 
there  lay  granules  of  slightly  greater  specific  gravity,  that  would  fall 
to  the  bottom  of  the  vacuole,  whatever  position  it  occupied,  and  rest 
against  the  cytoplasmic  membrane  bounding  it.  In  the  normal  position 
of  parallelotropic  organs  this  would  lead  to  no  excitation;  but  if  the  cells 


GROWTH    AND    MOVEMENT 


465 


were  displaced,  the  granules  would  settle  upon  a  new  and  excitable  side 

of  the  vacuole  wall,  starting  into  action  the  mechanism  of  the  end  re- 
sponse. 

There  are  many  objections  l<>  this  form  of  the  theory,  which  was  suggested  by  the 
visible  otocysts  of  Crustacea,  and  the  appearance  of  the  centrosomes,  which  were 

then  supposed  to  be  common  in  the  cells  of  seed  plants. 

In  a  more  concrete  form  the  theory  has  much  to  commend  it,  though 
it  cannot  yet  be  considered  as  firmly  established.  In  this  form  no  in- 
visible structures  are  predicated,  but  the  principle  is  the  same.     Certain 


Figs.  697,  6q8. —  Perceptive  regions:  697,  median  longitudinal  section  of  the  rootcap 
of  Roripa  amphibia  ;  d,  dermatogen;  698,  apex  of  the  coleoptile  of  the  plumule  of 
Panicum  miliaceum.  —  After  Ni'.mi.c. 


cells,  notably  those  of  the  inner  median  portions  of  the  root  cap  (fig.  697), 
the  tip  of  the  coleoptile  in  grasses  (fig.  698),  and  a  layer  around  the  vas- 
cular cylinder  in  stems,  contain  rather  large  starch  grains  in  such  abun- 
dance as  to  attract  attention.  Moreover,  these  starch  grains  are  freely 
movable,  and  in  whatever  position  the  organ  rests  they  accumulate  on 
the  physically  lower  side  of  the  tells.  They  seemed  to  answer  the  re- 
quirement for  bodies  Heavier  than  the  fluid  in  which  they  lie,  and  there- 
fore capable  of  setting  up  an  excitation  by  coining  to  rest  upon  a  part  of 
the  protoplast  unaccustomed  to  their  contact.  It  is  assumed  thai  cer- 
tain areas  of  the  protoplast  are  properly  sensitive;  thai  their  excitation 
will  start  into  activity  the  mechanism  of  curvature,  which  will  eventually 
restore  the  organ  to  its  normal  position  and  so  remove  the  irritating  starch 

C.  B.  &  C.  HOT  ANY  —  30 


466  PHYSIOLOGY 

grains  from  excitable  areas,  tumbling  them  again  upon  the  side  corre- 
sponding to  the  position  of  equilibrium. 

These  mobile  grains  are  called  statoliths  and  the  cells  containing  them  statocysts, 
after  the  analogy  of  the  otocysts  of  the  Crustacea,  once  thought  to  be  organs  of  hear- 
ing, but  now  recognized  as  organs  of  equilibrium.  The  semicircular  canals  of  the 
ear  of  vertebrates,  with  their  fluid  and  mineral  granules,  have  a  similar  function, 
giving  the  animal  a  sense  of  position  or  equilibrium. 

Extensive  anatomical  studies  have  shown  a  remarkable  parallelism  between  the 
presence  of  such  grains  and  geotropic  sensitiveness.  Almost  without  exception, 
geotropic  organs  have  mobile  starch  grains,  while  non-geotropic  organs  lack  them. 
Moreover,  when  an  organ,  placed  under  unfavorable  conditions  (e.g.  low  tem- 
perature), has  lost  its  mobile  starch,  it  seems  at  the  same  time  to  have  lost  its  geo- 
tropism,  which  is  regained  simultaneously  with  the  rebuilding  of  the  starch  grains 
and  not  until  then,  although  conditions  favorable  for  response  (had  perception  been 
possible)  may  have  existed  for  much  longer  than  the  usual  reaction  time.  This 
method  of  experimentation  is,  indeed,  open  to  some  objections  ;  but  the  most  serious 
one,  namely,  that  the  unfavorable  temperature  which  determines  the  removal  of 
the  starch  at  the  same  time  suppresses  the  geotropic  irritability,  is  largely  obviated 
by  the  fact  that  perception  and  the  end  reaction  can  be  separately  interfered  with. 
Thus,  by  a  temporary  reduction  of  temperature,  perception  is  not  interfered  with, 
for  upon  again  raising  the  temperature  with  no  further  stimulation  the  end  reaction 
proceeds  as  usual.  Further,  it  is  executed  more  promptly  after  the  restoration  of 
favorable  temperature  than  it  is  when  the  low  temperature  is  first  used  to  eliminate 
the  starch,  and  then  at  a  favorable  temperature  stimulation  is  attempted.  This  in- 
dicates that  the  failure  to  obtain  the  curvature  when  there  is  no  mobile  starch  is 
due  to  an  interference  with  the  mechanism  of  perception  rather  than  with  the 
mechanism  of  transmission  or  of  growth. 

Plagiotropic  organs.  —  The  erect  position  of  certain  organs  is  not 
necessarily  determined  by  gravity  alone,  but  may  be  due  to  the  coopera- 
tive action  of  other  stimuli.  In  like  manner  the  oblique  or  horizontal 
position  may  be  determined  wholly  by  a  response  to  gravity,  or  by  some 
other  single  stimulus,  or  by  simultaneously  acting  stimuli.  Experiment 
alone  can  determine  the  agents  in  each  case.  Among  plagiotropic  or- 
gans which  owe  their  position  to  gravity,  some  rhizomes  that  run  hori- 
zontally beneath  the  surface  of  the  ground  are  noteworthy.  When  such 
a  rootstock  is  displaced  by  directing  the  tip  obliquely  upward  or  down- 
ward, curvatures  ensue,  precisely  as  in  the  case  of  parallelotropic  roots, 
though,  of  course,  the  growth  is  much  slower.  This  mode  of  reaction  is 
known  as  transverse  geotropism  or  diageotropism,  corresponding  to  the 
positive  and  negative  geotropism  of  parallelotropic  organs.  Quite 
similar  behavior  is  to  be  seen  in  some  peduncles,  which  are  pendulous 
while  the  flower  is  in  bud,  but  become  in  bloom  horizontal,  and  in  fruit 


GROWTH    AND    M<»\  I  MEN  I'  407 

erect  (figs.  1105-1197).  When  the  change  of  position  can  be  shown  to  be 
due  wholly  to  gravity,  this  indicates  that  the  peduncle  undergoes  with 
age  a  change  in  its  mode  of  response.  Well  known  examples  are  offered 
by  the  snowdrop  and  the  wind  flower.  Less  generally  known  are  like 
changes  in  direction  when  certain  stems,  erect  in  the  seedling  stage, 
develop  into  horizontal  rhizomes  in  an  older  stage. 

Diageotropism.  —  Diageotropism  of  a  somewhat  modified  type  is  seen 
in  the  branches  of  the  primary  roots  of  some  plants.  These  grow  out 
at  a  definite  angle,  and,  if  displaced,  they  will  curve  until  the  normal 
angle  is  again  attained.  Similarly  the  oblique  branches  of  trees  some 
times  are  decidedly  geotropic,  and  even  the  pendent  ones  may  show  it. 
Only  by  the  most  cautious  and  precise  experimentation  in  each  «  ase 
can  it  be  ascertained  whether  the  positions  assumed  are  due  to  gravity. 
Unwarranted  generalizations  in  this  direction  are  particularly  seductive. 
In  far  the  greater  number  of  cases  the  position  of  organs  is  determined 
by  a  complex  of  stimuli  most  difficult  of  analysis. 

Twiners.  —  Among  the  most  interesting  of  the  complex  phenomena 
are  those  exhibited  by  twining  plants,  in  which  geotropic  reaction  of  a 
peculiar  kind  plays  a  most  important  part.  Twiners  have  slender  stems 
with  a  very  long  growing  region,  and  a  tardy  development  of  the  lateral 
organs  (leaves  and  branches),  so  that  the  long  tips  often  look  quite 
naked.  These  ends  seem  to  travel  in  a  spiral  fashion  around  some  suit- 
able, slender  support,  and  the  mature  plant  is  thus  wound  around  it 
and  clasps  it  tightly.  At  the  outset  the  seedling,  say  of  a  morning  glory, 
grows  quite  erect,  and  seems  like  a  parallelotropic  plant,  as,  indeed,  a 
study  of  its  reactions  with  a  clinostat  shows  it  to  be  at  this  period.  After 
reaching  a  certain  height  the  tip  no  longer  grows  erect,  but  declines  to 
one  side,  and  then  a  movement  begins, quite  like  the  irregular  nutation 
that  every  erect  plant  makes,  except  that  it  is  regular  and  more  striking. 
The  tip,  standing  in  a  nearly  horizontal  line,  swings  steadily  around  and 
is  directed  successively  to  every  point  of  the  compass.  This  may  bring 
it  into  contact  with  a  suitable  support,  around  which  it  then  proceeds  to 
twine,  the  free  tip  continuing  the  swinging  movement  from  the  point  of 
contact  with  the  support.  The  fundamental  feature  of  the  twining, 
therefore,  is  the  swinging  motion. 

Lateral  geotropism.  —  Since  the  swinging  movement  does  not  con- 
tinue when  'a  twiner  is  properly  rotated  on  a  clinostat,  it  must  be  con- 
sidered a  response  to  gravity.  As  growth  thai  can  swing  the  tip  sidewise 
can  be  effective  only  if  it  takes  place  on  the  Hank,  the  inference  is  made 


468  PHYSIOLOGY 

that  the  stimulus,  instead  of  finally  affecting  the  side  of  the  stem  next 
the  earth,  as  it  does  in  the  younger  stages  of  development,  now  affects 
the  flank,  determining  there  more  rapid  growth.  According  as  the  right 
flank  or  the  left  grows  faster,  the  tip  will  be  swung  like  the  hands  of  a 
clock  or  in  the  opposite  direction.  The  twining  may  then  be  desig- 
nated as  clockwise  or  counter-clockwise  (see  Part  III,  fig.  957).  There 
is  no  fundamental  reason,  apparently,  for  one  direction  rather  than 
the  other.  While  usually  the  same  species  of  plant  twines  always  in 
the  same  fashion,  closely  allied  species  will  differ  in  this;  there  are  some 
species  that  twine  indifferently  in  either  direction;  and  there  are  a  few 
in  which  the  individual  plant  may  change  the  direction  of  twining  in 
the  course  of  its  development. 

Rotation  and  revolution.  —  When  growth  of  a  given  flank  has  swung 
the  free  tip  around,  this  very  act,  by  twisting  the  stem  on  its  own  axis, 
brings  a  new  segment  of  the  stem  into  the  flank  position  and  so  exposes 
it  to  excitation. 

This  may  be  understood  by  representing  the  stem  by  a  hexagonal  pencil.  If 
the  side  on  which  the  name  is  stamped  face  the  right  with  the  pencil  horizontal 
and  the  point  away  from  the  body,  then  this  right  flank  may  be  imagined  to  be  the 
one  whose  growth  is  accelerated;  by  that  the  point  would  be  swung  to  the  left,  and 
by  the  time  it  has  passed  over  oo°  the  pencil  would  be  rotated  on  its  axis  through 
90°,  so  that  the  stamped  side  would  now  face  upward  and  the  angle  that  was  first  at 
the  bottom  would  now  be  the  flank.  This  rotation  may  be  imitated,  if  it  cannot  be 
seen  to  be  a  mechanical  necessity  when  a  horizontal  portion  of  an  erect  stem  is  so 
rotated,  by  sticking  the  end  of  a  pencil  into  a  piece  of  rubber  tubing  just  stiff  enough 
to  bend  into  a  quadrant  under  its  weight.  Now  upon  swinging  this  apparatus  without 
torsion,  as  can  be  done  by  holding  the  end  of  the  tube  and  pushing  the  test  pencil 
around  with  another,  the  rotation  will  become  at  once  evident,  being  complete  when 
one  revolution  is  completed. 

The  new  flank  thus  brought  under  the  influence  of  gravity  has  its  rate 
of  growth  increased,  which  swings  the  tip  further,  rotates  the  free  part 
of  the  axis,  and  so  brings  another  segment  into  the  flank  position.  Given 
the  sensitiveness  of  the  flank  to  gravity,  the  revolving  movement  follows 
as  a  necessity. 

The  support.  —  When  a  stem  is  swinging  thus,  if  it  come  into  contact 
with  some  obstacle  near  the  tip,  flexure  may  carry  it  past  the  object ; 
but  if  it  strikes  the  obstruction  further  back,  the  bending  may  not  be 
sufficient  to  carry  the  axis  past  the  obstacle,  particularly  if  it  be  of  moder- 
ate size.  Instead,  curvature  will  soon  occur  in  the  part  projecting 
beyond  it,  and  the  revolving  movement  will  be  continued  by  the  apical 


GROWTH    AND    MOVEMENT  469 

portion,  which  steadily  wraps  itself  around  the  support.  In  the  nature 
of  the  case  it  is  not  possible  for  twiners  to  wrap  about  large  supports, 
nor  those  that  are  too  nearly  horizontal.  Plants  differ  much  in  their 
capacity  in  these  two  poim-,  a  difference  which  depends  chiefly  upon  the 
relative  length  of  the  growing  portion  of  their  stems,  and  consequently 
upon  the  precise  distance  of  the  most  actively  growing  region  from  the 
apex.  Few  twiners  encircle  supports  more  than  15  cm.  in  diameter,  or 
those  that  lie  nearer  the  level  than  450. 

Straightening.  —  The  coils  that  a  twiner  forms  at  first  are  loose  and  of 
low  inclination.  Later  they  become  steeper  and  hug  the  support  tightly. 
This  seems  to  be  due  to  a  return,  in  the  last  stages  of  growth,  to  the 
apogeotropism  that  they  possessed  in  the  seedling  period,  so  that  the 
stem  starts  to  erect  itself,  with  the  effect  stated.  Very  commonly  the 
surface  of  the  stem  is  rough,  being  ridged  or  angled  or  furnished  with 
stiff  hairs,  which  prevents  slipping  from  a  support  too  easily  or  sliding 
along  it.  Inspection  of  the  stem  in  the  regions  no  longer  growing  shows 
that  it  is  twisted,  the  longitudinal  ridges  coursing  spirally  around  the 
axis  in  a  direction  the  reverse  of  the  twining.  This  torsion  is  mainly 
the  result  of  the  final  erection  of  the  stem,  though  other  causes  cooperate 
to  increase  or  diminish  it. 

This  also  is  a  mechanical  necessity  of  the  behavior.  It  can  be  imitated  by  coiling 
a  long  piece  of  rubber  tubing  on  a  tabic,  marking  a  crayon  line  along  the  upper 
surface,  and  then  lifting  the  inner  end  of  the  (  oil  while  the  other  end  is  held  on  the 
table,  both  ends  being  prevented  from  twisting  in  the  fingers.  Then  it  will  be  seen 
that  the  line  apparently  passes  spirally  around  the  tube,  because  the  latter  is  twisted 
by  the  steepening  of  its  coils. 

The  tardy  development  of  the  leaves  and  branches  is  very  evidently 
an  advantage  in  twining,  for  they  would  greatly  impede  the  revolving 
movement  and  the  subsequent  tightening  of  the  coils.  When  tin- 
branches  do  develop,  they  show  the  same  behavior  as  the  main  axis. 

This  explanation  of  twining  is  not  wholly  satisfactory,  because  there  are  details 

of  the  process,  and  some  features  that  appear  only  under  experiment,  that  are  no! 

clearly  accounted  for ;  but  it  is  far  the  best  of  the  many  theories  that  have  been  pro- 
posed, and  in  the  major  outlines  that  have  been  presented  here  it  iscertainU 

(2)     THIGMOTROPISM 

Tendrils.. —  Many  plant-  are  sensitive  to  mechanical  stimuli  such  as 
contact  <>r  friction,  as  shown  by  the  alteration-  of  the  rate  of  growth 
thai    lead    to   curvature.    This   phenomenon    i-   tkigmotropism.     The 


470      _  PHYSIOLOGY 

tendrils  of  climbing  plants  exhibit  the  most  remarkable  sensitiveness  to 
mechanical  stimuli,  and  it  is  by  this  means  that  their  attachment  to 
supports  is  secured.  Tendrils  are  slender,  even  threadlike,  lateral  or- 
gans, branched  or  not,  sometimes  occupying  the  usual  place  of  a  branch, 
sometimes  that  of  a  leaf  or  of  one  or  more  leaflets  of  a  compound  leaf. 
They  are  therefore  formed  successively  with  the  development  of  the 
main  axis  and  its  chief  branches,  so  that  the  plant  is  constantly  laying 
hold  of  a  support  by  younger  and  younger  tendrils.  It  may  thus  climb 
to  great  heights,  while  the  main  axes  remain  very  slender  and  wholly 
unable  to  support  their  own  weight,  much  less  that  of  foliage,  flowers, 
and  fruit.  The  most  important  feature  of  the  tendril  is  its  irritability 
to  contact,  and  the  curvatures  which  follow  as  end  reactions. 

Behavior.  —  When  a  tendril  is  young  and  only  about  one  fourth  grown, 
it  may  be  either  straight  or  curled  up  into  a  loose  spiral,  of  which  the 
convex  surface  corresponds  to  the  under  side.  If  coiled,  it  unrolls  as 
its  period  of  rapid  growth  begins,  at  which  time  also  begin  nutating 
movements  that  are  almost  as  regular  as  the  revolving  movements  just  de- 
scribed in  the  twiners.  These  tendril  movements,  however,  are  not  due  to 
any  known  external  stimulus,  but  must  be  called  at  present  autonomic. 
The  tip  is  thereby  swung  in  all  directions  and  is  thus  likely  to  come  into 
contact  with  some  suitable  support.  When  it  does  so,  it  quickly  wraps 
around  it.  After  a  time,  through  continued  and  unequal  growth  in 
length,  spiral  coils  appear  in  the  region  between  the  axis  and  the  attached 
part,  increase  in  number  and  closeness,  and  become  more  and  more  firm, 
until  this  part  has  become  a  veritable  spiral  spring  by  which  the  plant 
is  slung  to  its  support.    These  results  are  attained  in  the  following  way: 

Stimulus  :  friction.  —  The  tendril  is  sensitive  to  contact,  usually 
throughout  its  whole  length,  and  on  all  sides,  but  most  so  towards  the 
tip.  Yet  it  is  not  sensitive  to  contact  in  the  narrow  sense;  it  is  because 
things  come  into  contact  with  the  tendril  in  more  than  one  place  when 
they  touch,  so  that  it  is  only  by  multiple  and  successive  contacts,  and 
usually  by  shifting  contact  or  friction,  that  the  tendril  is  excited.  Liquids 
(even  the  heaviest,  mercury),  if  entirely  free  from  solid  particles,  and 
perfectly  smooth  solids,  like  gelatin,  do  not  produce  excitation.  Rain, 
therefore,  does  not  cause  useless  movements  of  tendrils.  But  very  slight 
rubbing  movements  of  excessively  light  objects  suffice  to  start  them. 
It  has  been  found,  for  example,  that  a  bit  of  thread,  weighing  by  estimate 
only  0.00025  mg.,  if  moved  by  the  wind  over  a  very  sensitive  tendril,  will 
induce  curvature. 


CROW  III    AND    MOVEMENT 


471 


While  the  tendril  may  be  sensitive  throughout,  the  responses  evoked  by  excita- 
tion ditTcr  sometimes  according  to  the  region  stimulated.  Thus,  a  stimulus  applied 
to  the  "  under  "  side,  which  at  the  time  <>f  greatest  sensitiveness  has  usually  grown 
near  the  apex  a  little  less  than  the  other,  so  that  at  the  tip  it  is  slightly  concave, 
results  in  a  curvature.  So  also  does  stimulation  of  the  Banks,  and  in  some  tendrils 
that  of  the  upper  side  too.  But  there  are  some  others  whi<  h  give  no  sign  if  rubbed 
on  the  upper  side,  except  that  stimulation  there  will  inhibit  a  simultaneous  stimula- 
tion on  the  under  side,  which  ordinarily  would  result  in  a  curvature. 

Primary  response.  —  The  first  result  of  slight  rubbing  contact  with  a 
suitable  support  (that  is,  one  that  is  small  enough  fur  the  tendril  to  en- 
circle, no  matter  in  what  position  it  stands)  is  a  prompt  curvature.  In 
sensitive  tendrils  under  favorable  conditions  this  follows  in  the  course 
of  a  few  seconds  (5-30),  but  in  others  in  a  few  minutes.  The  facts  ob- 
served are  that  the  cells  on  the  convex  side  become  suddenly  consider- 
ably elongated,  while  those  on  the  concave  side  become  somewhat  short- 
ened. This  and  the  promptness  of  the  end  reaction  suggest  a  turgor 
change,  and  many  observers  have  concluded  that  such  is  the  mechanism 
of  the  primary  curvature,  and  that  it  becomes  fixed  later  by  growth. 
Others  attribute  these  results  to  a  very  rapid  and  extraordinarily  sudden 
growth  of  the  cells  of  the  convex  side,  and  to  the  consequent  compression 
of  those  on  the  concave  side.  It  is  not  improbable  that  the  truth  in  this, 
as  in  many  similar  recondite  and  much  controverted  matters,  will  prove 
to  lie  between  the  contentions.  So  it  may  very  well  be  that  a  turgor  varia- 
tion begins  the  movement,  whereupon  growth  follows  it  up  more  promptly 
than  usual,  and  extends  and  completes  the  encircling  of  the  support. 

Secondary  response.  —  After  the  tendril  has  become  firmly  attached, 
the  excitation  extends  toward  the  base  of  the  tendril,  producing  an  in- 
equality of  growth  on  the  opposite  sides  (in  this  case  the  "  upper  "  side 
becomes  the  convex  one)  that  throws  this  part  of  the  tendril  into  coils 
(see  Part  III,  fig.  958). 

This  coiling  may  be  rudely  but  essentially  imitated  by  placing  in  a  pan  of  water 
a  narrow  strip,  slit  from  the  scape  of  a  fruiting  dandelion  which  has  not  attained  it. 
full  height,  and  by  pinching  eat  h  end  in  a  short  folded  piece  of  sheet  lead  to  prevent 
twisting.  After  a  few  hours  the  strip  will  be  found  coiled  into  a  spiral,  with  one  or 
more  reversals  of  direction  just  as  in  the  tendril,  though  more  irregularly.  Here 
the  tissue  next  the  pith  cavity  grows  and  becomes  more  turgid  than  the  epidermal 
and  cortical  tissues.  The  reversal  of  coil  is  a  mechanical  necessity  if  the  ends  are 
not  free  to  rotate. 

These  coils  are  not  merely  the  result  of  continued  growth  of  the  tendril; 
for  if  one  not  full  grown  becomes  attached,  it  does  nol  rea<  It  it-  possible 


472  PHYSIOLOGY 

maximum  length,  but  from  that  time  grows  only  in  such  a  way  as  to 
throw  it  into  the  spiral  coils.  One  which  does  not  become  attached  grows 
longer  and  longer,  but  finally  shrivels,  usually  without  coiling.  Soonei 
or  later,  upon  the  cessation  of  this  second  phase  of  growth,  the  phase  of 
maturation  is  marked  by  the  development  of  mechanical  tissues,  which 
add  strength  to  the  elastic  coils.  The  nature  of  the  stimulus  that  brings 
about  the  final  coiling  is  uncertain.  It  may  be  the  strain  from  the  weight 
of  the  plant  after  becoming  fastened,  or  the  spreading  stimulation  from 
the  contact  pressure  (for  the  attachment  coils  compress  the  support), 
or  some  unsuspected  stimulus  may  be  brought  into  action.  There  are 
many  other  stimuli  which  will  evoke  reactions  from  the  tendrils,  but 
none  which  in  nature  has  any  importance. 

Sensitive  petioles.  — There  are  other  plant  organs  that  behave  in  a  similar  way 
to  the  tendrils,  though  none  of  them  is  so  sensitive.  The  petioles  of  Clematis  and 
of  the  climbing  Tropaeolum,  or  "nasturtium,"  are  familiar  examples.  While  such 
petioles  do  not  wrap  themselves  around  the  support  nor  form  spiral  coils  as  well  as 
a  tendril  does,  nevertheless  they  are  efficient  prehensile  organs,  enabling  the  plants 
to  climb  high. 

Dodders.  —  Any  account  of  twining  and  climbing  plants  would  be 
incomplete  without  mention  of  the  dodders  (Ciiscuta),  leafless  yellowish 
parasites  that  wind  their  stems  around  and  clamber  over  erect  herbaceous 
plants,  sending  haustoria  into  their  stems,  whence  they  obtain  food  and 
water.  In  the  first  stages  of  development,  the  species  that  have  been 
studied  germinate  in  the  soil,  and  the  young  seedling  behaves  as  a  twiner; 
but  shortly  after  it  has  found  a  suitable  host  and  begun  to  twine  around  it, 
the  lower  part  of  the  stem  dies  away,  while  the  upper  part  continues  its 
growth  at  the  expense  of  the  host.  The  further  twining,  however,  in- 
stead of  being  dependent  upon  gravity,  is  the  result  of  a  contact  stimulus 
like  that  which  enables  tendrils  to  secure  a  hold,  so  that  the  parasite 
enwraps  supports  in  all  sorts  of  positions.  In  the  possession  of  these 
two  modes  of  response  at  different  periods  of  development,  the  dodder? 
are  unique  (see  further  Part  III,  fig.  io8t). 

(3)  Traumatropism 
The  wounding  of  plants  produces  immediate  reactions,  mostly  invisible,  but  root 
tips  may  be  so  wounded  as  to  lead  to  curvature.  If  an  active  tip  be  branded  on 
one  side  with  a  hot  iron  or  glass  rod,  or  if  it  be  similarly  cut  or  otherwise  injured,  the 
tip  will  turn  to  one  side.  When  the  injury  is  severe,  this  may  so  seriously  impair  the 
tissues  on  the  injured  side  that  their  growth  will  cease,  and  the  injured  side  will 
become  concave  near  the  point  of  injury,  because  there  the  tissues  shrivel  and  the 
growth  of  the  other  side  goes  on.    This  is  not  a  true  reaction,  since  the  result  is 


GROWTH    AND    MOVEMENT  473 

due  merely  to  mechanical  interference  with  growth.  On  the  other  hand,  if  the  in- 
jurs- is  one  that  <1<ics  not  deeply  involve  the  tissues  of  the  injured  side,  a  1  urvature 
will  follow  that  turns  the  tip  away  from  the  injury.  Here  an  excitation  started  l>y 
the  wound  lias  spread  thence  to  the  region  of  most  rapid  growth,  inducing  a  true 
tropic  curvature.  After  experiments  by  attaching  bits  of  cinder,  paper,  and  the 
like  to  root  tips  by  means  of  gum,  ii  was  believed  that  the  root  tip,  by  its  sensitive- 
ness to  contact,  was  a  sort  of  direi  live  organ,  whii  b  1  ould  feel  its  way  through  the 
soil,  and  avoid  injury.  I'.ut  in  these  experiments  the  gum  injured  the  cells,  and  it, 
not  the  attached  particle,  was  the  stimulating  agent,  so  that  the  response  was  ac- 
tually to  injury  and  not  to  contact.  It  is  not  probable  that  sensitiveness  to  injury 
is  of  any  advantage  to  the  plant,  as  it  undoubtedly  is  to  a  conscious  organism. 
( Occasionally,  of  course,  traumatropism  might  he  advantageous  to  a  plant  in  getting 
a  root  tip  once  injured  out  of  immediate  danger  of  further  injury. 

(4)   Rheotropism 

Roots  grown  in  a  current  of  water  of  adequate  veloi  it  y  may  respond  by  directing 
their  tips  against  the  current.  In  this  case  the  stimulus  might  he  the  strains  set 
up  by  the  pressure  of  the  current,  or  the  impact  and  friction  of  the  water  parti*  les 
against  the  surface.  Its  precise  nature  is  not  satisfactorily  determined,  hut  it  seems 
to  be  the  pressure  of  the  water  and  the  resulting  strains  rather  than  mere  contai  t 
or  impact.  The  whole  of  the  growing  region  seems  to  he  sensitise,  and  not  the  tip 
ali  tne.  It  is  not  apparent  that  this  reaction  can  have  any  significance  for  the  plant  in 
nature. 

(5)  Chemotropism 

Of  fungi.  —  Chemical  compounds  may  not  only  be  usable  in  repair 
and  constructive  work,  but  may  so  affect  the  living  substance  and  its 
chemism  as  to  act  upon  it  as  stimuli.  Since  by  diffusion  they  may  act 
from  one  side,  these  stimuli  may  be  directive,  causing  curvatures  toward 
or  away  from  the  source,  which  are  manifestations  of  chemotropism. 
Very  striking  reactions  to  chemical  compounds  of  many  sorts  have  been 
ascribed  to  the  hyphae  of  fungi  and  to  pollen  tubes.  Chemotropism  of 
the  latter  may  be  maintained  still,  as  it  has  not  been  seriously  im- 
peached; but  that  of  fungus  hyphae  has  been  brought  under  suspicion 
by  the  latest  researches,  and  may  be  cither  established  or  disproved  by 
further  study.  For  the  hyphae  to  be  sensitive,  especially  to  carbohy- 
drate and  other  foods,  would  be  of  much  service  in  inducing  them  to 
grow  in  directions  that  would  bring  them  into  favorable  feeding  regions, 
and  precisely  this  power  has  been  as<  ribed  to  them.  For  instance,  when 
certain  fungus  spores  are  sown  in  a  layer  of  gelatin  containing  no  nutritive 
materials,  between  layers  of  gelatin,  on  the  one  side  with  nutritive  ma- 
terial and  on  the  other  side  without,  it   is  reported   that  the  hyphae  turn 

toward  the  layer  of  nutritive  gelatin.    The  same  reaction  was  found  to 


474  PHYSIOLOGY 

occur  when  the  central  layer  contained  food,  provided  the  outer  layer 
had  enough  more  of  the  same  to  act  as  a  stimulus.  (In  this  case  the  ratio 
had  to  be  about  10  :  i.  See  Weber's  law,  p.  448.)  Likewise  the  hyphae 
grew  through  fine  perforations  in  thin  plates  of  mica  or  celluloid,  when 
the  nutritive  gelatin  was  thus  separated  from  the  other,  suggesting  the 
way  in  which  fungus  hyphae,  arising  from  spores  on  a  leaf,  turn  into  a 
stoma  and  so  find  their  way  into  the  interior  of  a  leaf  of  their  host.  In 
fact,  when  leaves  were  injected  with  a  solution  of  food,  like  sugar,  fungus 
hyphae  of  many  kinds  are  reported  to  turn  into  the  stomata,  though  they 
do  not  naturally  grow  on  the  leaves  used.  A  great  variety  of  substances 
were  tested  in  similar  ways.  Some  proved  to  be  attractive,  some  repel- 
lent; and  the  reaction  varied  according  to  the  concentration  of  the  solute, 
though  generally  the  hyphae  were  injured  before  the  limits  of  concen- 
tration for  repelling  effects  had  been  reached. 

On  the  other  hand,  an  apparently  careful  repetition  of  many  of  these 
experiments  gave  negative  results,  in  that  the  numbers  of  hyphae  reacting 
positively  is  so  slightly  in  excess  of  the  number  indifferent  or  negative, 
that  the  results  seem  scarcely  more  than  chance,  or  ascribable  to  other 
than  the  cause  assigned  heretofore.  A  complete  restudy  of  the  matter 
will  be  necessary. 

Of  pollen  tubes.  —  When  pollen  tubes  are  developed  under  a  cover 
glass  in  company  with  a  bit  of  the  stigma  of  the  same  plant,  they  turn 
toward  it,  from  whatever  direction  they  first  issue.  An  ovule  or  a  bit 
of  the  wall  of  the  ovary  is  likewise  attractive.  Investigation  shows  that 
soluble  carbohydrates  and  proteins  are  here  the  attractive  substances. 
It  seems  likely,  therefore,  that  the  growth  of  the  pollen  tube  toward  the 
ovules  is  directed  by  the  diffusion  of  such  substances,  which  are  always 
found  in  these  organs.     (See  the  chemotaxy  of  sperms,  p.  448.) 

Aerotropism.  —  A  special  form  of  chemotropism  has  been  called  aerotropism, 
and  was  first  ascribed  to  roots.  When  certain  gases,  especially  oxygen,  diffuse 
against  young  roots  from  one  side,  it  is  reported  that  the  root  curves  toward  the 
Source  of  the  gas.  These  results  also  have  fallen  under  suspicion.  Recent  investi- 
gations are  conflicting ;  and  one  is  left  in  some  doubt  whether  to  ascribe  the  curva- 
tures to  a  true  reaction  to  gases,  in  accordance  with  the  weight  of  evidence,  or  to 
moisture,  in  which  case  they  belong  to  the  following  special  category  of  chemotropic 
response. 

Stems  also  have  shown  sensitiveness  to  O2  and  CO2,  and  it  may  be  that  aero- 
tropism is  more  general  than  has  heretofore  appeared.  It  is  not  evident  that  it 
can  be  of  any  great  advantage  to  either  roots  or  stems,  except,  perhaps,  those  of 
swamp  plants. 


GROWTH    AND    MOVEMENT  475 

Hydrotropism.  —  Another  special  form  of  chemotropism,  which  has 
been  named   hydrotropism,  designates  the  sensitiveness  of  root-,   the 

hyphae  of  some  fungi,  the  rhizoids  of  liverwort-,  et<  .,  exhibited  by  turn- 
ing toward  or  away  from  the  source  of  diffusing  water  vapor,  or  capil- 
lary water  in  soils.  When  seedlings  are  grown  in  an  atmosphere  less 
than  saturated  with  water  vapor,  so  that  the  roots,  as  they  grow,  pass 
further  and  further  away  from  a  wet  surface,1  it  will  be  found  that  they 
deviate  presently  from  the  perpendicular,  inclining  toward  the  wet 
surface;  soon  again  they  turn  downwards,  but  once  more  return  to  the 
moisture,  and  this  may  be  repeated  many  times.  Plainly  the  root-  are 
subject  here  to  two  stimuli  acting  nearly  at  right  angles,  gravity  and  the 
diffusing  vapor.  First  the  one  dominates  and  then  the  other.  Were  it 
not  for  the  long  reaction  times  the  root  might  be  expected  to  take  an  inter- 
mediate direction,  the  resultant  of  the  effects  of  the  two  stimuli;  but  as 
in  the  case  of  gcotropism  alone  (see  p.  460),  the  after-effects  carry  the  root 
tip  past  the  position  of  equilibrium,  whereupon  the  other  stimulus  gives 
it  such  strong  and  long  excitation  that  its  after-effects  carry  the  root 
again  past  the  equilibrium  point;  then  the  gravity  stimulus  comes  upon 
it  again;  and  so  it  weaves  back  and  forth. 

The  vegetative  hyphae  of  the  mold  fungi  may  show  positive  hydro- 
tropism and  their  sporangiophores  negative  hydrotropism.  It  can 
easily  be  shown  that  the  rhizoids  of  Marchantia,  which  normally  grow 
straight  downward,  will  deviate  toward  a  moist  surface  in  the  same  way 
as  roots;  only  the  moisture  stimulus  is  dominant  over  gravity.  Roots 
in  the  soils  also  grow  towards  the  moister  regions,  and  especially  do  they 
tend  toward  tile  drains,  into  which  they  may  penetrate,  often  branching 
profusely  enough  to  plug  up  the  drain  completely.  Part  of  this  direc- 
tive effect  may  be  due,  and  probably  most  of  the  branching  is  due,2  to 
chemical  stimulation  by  the  solutes. 

(6)   Phototropism 

Stimulus.  —  Of  all  the  external  conditions  that  act  upon  plants,  light 
is  one  of  the  most  variable,  for  from  time  to  time  it  differs  in  direction, 
in  intensity,  and  in  quality.  Quite  apart  from  its  fundamental  relation 
to  all  life  jn  furnishing  the  energy  for  food  making,  arc  it-  effects  as  a 
stimulus.     Whereas  the  most  effective  quality  of  light  for  food  making 

1  As  by  planting  them  in  <  oarse  sawdust  held  in  plat  e  on  the  under  Burfai  e  of  an  ini  lined 
board  by  bobbinrt. 

2  In  which  case  this  is  .1  morphogenic  effect     See  p.  1.35. 


476  PHYSIOLOGY 

is  the  red-yellow,  the  most  effective  light  as  a  stimulus  is  that  near  the 
violet  end  of  the  spectrum.  Since  this  is  the  region  of  least  energy,  the 
shortness  and  frequency  of  the  waves  are  the  important  features  of  light 
as  a  stimulus.  In  this  respect  the  red  end  of  the  spectrum,  though  its 
energy  is  far  greater,  behaves  as  darkness. 

Response.  —  In  general  the  response  of  plants  to  light  differs  according 
to  the  usual  attitude  of  the  organ  and  its  mode  of  growth,  for  which 
indeed  light  is  largely  determinative.  Parallelotropic  organs  respond 
by  directing  their  tips  toward  or  away  from  the  source  of  light,  while 
plagiotropic  organs  place  themselves  more  or  less  at  right  angles  to  the 
direction  of  the  rays.  Primary  stems,  therefore,  are  mostly  positively 
phototropic,  and  some  roots,  particularly  aerial  roots,  are  negatively 
phototropic;  while  leaves  are  mostly  transversely  phototropic  or  diapho- 
to  tropic. 

These  phenomena  were  first  known  as  heliotropism,  etc.,  and  are  often  still  so 
called,  because  the  sun  in  nature  is  the  source  of  all  light.  It  seems  better,  however, 
to  use  the  wider  term,  since  plants  respond  in  the  same  way  to  artificial  light,  which 
is  so  largely  used  in  experimental  work.  The  general  result  of  these  reactions  is 
the  same  as  of  those  to  gravity,  so  far  as  the  same  organs  are  sensitive  to  both 
stimuli,  though  the  two  act  from  opposite  directions  in  nature. 

Intensity.  —  The  intensity  of  the  light  may  determine  either  a  positive 
or  a  negative  curvature,  and  within  certain  limits  between  these  two  there 
is  a  range  of  intensity  which  calls  forth  no  visible  reaction  ;  this  is  the 
point  of  phototropic  indifference.  It  is  by  no  means  the  point  of  no 
excitation.  At  high  intensities  that  call  forth  negative  curvature,  injury 
soon  appears.  Near  the  lower  limit  of  intensity  that  can  produce  an 
end  reaction,  plants  show  themselves  very  sensitive  to  light.  Thus, 
radish  seedlings  respond  to  the  light  of  a  single  candle  at  a  distance  of 
about  8  m.,  the  broad  bean  (Vicia  Faba)  at  22  m.,  and  a  cress  (Lepidium 
sativum)  at  about  55  m.  The  differences  that  plants  can  distinguish  are 
within  the  limits  of  error  for  the  unaided  eye,  and  are  not  very  easily 
distinguishable  even  with  the  photometer. 

Time  relations.  —  The  presentation  time,  of  course,  depends  upon  the 
intensity  of  light  used,  and  is  approximately  inversely  proportional  to  it. 
The  greatest  range  of  presentation  time  recorded  is  that  for  etiolated 
seedlings  of  oats,  being  0.001  second  with  light  intensity  of  26,520  Hefner 
candles,  and  13  hours  with  light  intensity  of  0.000439  Hefner  candle. 
Intermediate  light  intensities  give  corresponding  inverse  proportional 
intermediate  presentation  times.     As  a  rule  the  younger  an  organ  is,  the 


GROWTH    AND   MOVEMENT  477 

more  sensitive  it  is;  but  this  is  by  no  means  universally  true.  The  re- 
action time  varies  fn>m  a  few  minutes  in  some  hours,  depending  upon 
tlu-  temperature,  the  intensity  of  the  light,  and  the  general  condition  of 
the  plant. 

Reversal. — The  reactions  to  light  also  are  often  reversed  with  age. 
This  is  especially  seen  in  flower  Stalks,  which  at  the  time  of  blooming  are 
likely  to  be  positively  phototropic,  but  later,  during  the  ripening  of  the 
fruit,  many  become  negatively  phototropic,  carrying  the  fruit  under  the 
leaves  or  even  into  crevices  of  the  soil  or  rocks  on  which  the  species  grows. 

Mechanism.  —  The  mechanism  of  the  response  is  the  same  as  in  geo- 
tropism,  and  occurs  in  the  same  region;  namely,  that  of  most  active 
growth,  where  one  side  grows  more  rapidly  than  the  other,  leading  to  a 
curvature  whose  tendency  is  to  direct  the  axis  into  the  line  of  the  light 
rays.  This  inequality  of  growth  is  brought  about  by  its  acceleration  on 
the  convex  side  and  by  simultaneous  retardation  on  the  concave  side. 
These  changes  in  rate  are  not  due  to  the  fact  that  the  rate  of  growth 
is  retarded  by  light  (see  p.  435),  for  this  (apparently  applicable  to  posi- 
tive phototropism  and  once  an  accepted  explanation)  could  not  ac<  ount 
for  the  acceleration  on  the  convex  side,  nor  for  any  of  the  changes  in 
negative  phototropism.  The  reaction  is  determined  by  the  mechanism 
of  the  parts  concerned  and  not  by  the  direct  influence  of  the  stimulus. 

Perceptive  region.  —  In  many  phototropic  reactions  there  is  a  distinct 
perceptive  region,  a  propagation  of  the  excitation,  and  an  end  reaction 
in  a  different  region.  Thus  when  seedlings  of  millet  raised  in  the  dark 
are  exposed  to  lateral  illumination,  the  sharp  curvature  that  presently 
appears  in  the  axis  ("  hypocotyl "),  which  is  rapidly  growing,  can  be 
shown  by  appropriate  shading  to  owe  its  origin  to  the  stimulus  perceived 
by  the  leaf  at  the  tip  ("cotyledon  ")  and  not  to  excitation  of  the  axis 
itself.  In  a  similar  way  the  seedlings  of  oats  show  that  though  the  w  hole 
of  the  subaerial  part  is  sensitive  t<>  light,  the  tip  is  much  the  most  so, 
and  that  excitation,  spreading  thence  downward,  dominates  even  con- 
trary excitation  set  up  in  the  lower  parts. 

What  is  perceived? —  Nothing  is  known  as  to  the  mode  of  perception 
or  the  structure  of  the  perceptive  organ.  Indeed,  it  is  not  entirely  cer- 
tain what  sort  of  stimulus  the  plants  perceive;  whether  it  is  the  direc- 
tion of  the  rays,  that  is,  the-  line  of  propagation  of  the  waves,  or  whether 
it  is  inequality  of  the  illumination  of  different  -ides.  It  ha-  even  been 
suggested,  in  casting  about  for  something  tangible,  that  plants  distin- 
guish between  the  different  pressures  in  the  lighted  and  shaded  portions! 


478  PHYSIOLOGY 

It  has  been  shown  that  the  impact  of  the  ether  waves  of  full  sunlight  produces 
a  pressure  equal  to  about  half  a  milligram  per  square  meter.  In  a  seedling  of  oats 
at  this  rate  the  plant  would  have  to  be  sensitive  to  a  difference  of  five  millionths  of  a 
milligram  and  probably  to  one  tenth  of  this  infinitesimal  amount.  This  is  simply 
inconceivable! 

It  seems  most  likely  that  it  is  the  difference  in  the  lighting  that  is  per- 
ceived, for  the  intensity  of  the  stimulus  has  an  important  bearing  on 
the  form  of  the  reaction,  and  plants  are  able  to  respond  to  differences 
of  illumination  coming  from  different  sides  that  are  too  small  for  the  eye 
to  distinguish. 

Plagiotropic  organs.  —  The  behavior  of  plagiotropic  organs  toward 
light  is  especially  interesting,  because  it  seems  to  be  usually  of  the  very 
greatest  importance  for  the  welfare  of  the  plant  in  food  making  by  leaves, 
thalli,  etc.  The  fact  that  the  leaves  of  most  common  plants,  set  before 
a  window,  place  themselves  at  right  angles  to  the  incident  light,  attracts 
attention  at  once.  If  the  pots  be  turned  around,  the  position  of  the  leaf 
blades  will  soon  be  changed,  and  they  face  the  window  again.  Thus  the 
leaves  obviously  come  into  a  position  most  advantageous  for  receiving  the 
maximum  of  energy  for  photosynthesis.  The  corresponding  orientation 
in  the  open  shows  that  it  is  not  the  direct  sunlight  alone  to  which  the 
leaves  respond,  but  rather  what  may  be  distinguished  as  sky  light;  that  is, 
the  brightest  diffused  or  reflected  light.  Indeed  in  some  cases  the  direct 
sunlight  is  evidently  too  intense,  and  the  plane  of  the  blades  is  set  at  an 
angle  to  the  direct  light,  the  edge  in  some  plants  being  directed  upward. 

Compass  plants.  —  When  the  position  of  leaves  is  uniform  or  nearly  so,  and  cor- 
responds approximately  with  the  plane  of  the  principal  meridian,  the  plants  are 
known  as  compass  plants.  The  wild  lettuce,  Lactuca  Scariola,  is  the  most  widely 
distributed  of  these,  and  on  the  prairies  and  along  railways,  the  compass  plant, 
Silphium  laciniatum,  which  illustrates  the  habit  far  better,  is  common.  Other 
plants  in  this  and  other  countries  have  the  same  habit.  That  this  is  a  response 
to  intense  light  is  seen  easily  in  the  lettuce,  for  when  this  plant  grows  in  the  shade, 
its  meridional  position  is  not  assumed. 

Fixed  light  position.  —  The  reaction  of  a  leaf  to  light  can  occur  only 
while  it  (especially  the  petiole,  which  is  the  seat  of  most  curvatures)  is 
still  growing  or  capable  of  growing.  During  this  period  the  habitual 
responses  lead  finally  to  a  position  known  as  the  fixed  light  position,  a 
sort  of  resultant,  which  on  the  whole  gives  the  blade  the  most  advan- 
tageous illumination.  One  result  of  this  is  the  arrangement  of  blades 
in  such  a  way  as  to  avoid  shading  one  another.  This  produces  the 
so-called  leaf  mosaics  (see  Part  III,  p.  543.)     The  movements  of  the  leaf 


GROWTH    AND    Ml  >VEMENT 


479 


Fig.   <><)<)■      Ordinary   epidermis    and    "ocella1 
(c)  of  leaf  of  Dioscorca.  — -After  Habkri.andt. 


in  attaining  these  positions  may  involve  curvature,  lengthening,  and 
twisting  of  the  petiole  and  even  of  the  blade  itself. 

Perceptive  region.-    Perception  in  most  cases    eems  i cur  in  the 

blade,  whence  the  excitation  is  propagated  to  the  petiole,  whose  upper 
parts  grow  for  the  longest  time,  and  even  after  elongation  has  ceased 
may  be  started  into  growth  again  by  the  light.  In  some  cases,  however, 
the  petiole  itself  may  be  sensitive  to  light,  and  may  either  cooperate  with 
the  blade,  or  alone  be  responsible  for  both  perception  and  curvature. 

The  mechanism  of  perception  has  been  sought  in  the  epidermis  of  the  blades. 
It  lias  been  found  in  some  cases  that  the  epidermal  cells  are  domed  and  that  they 
act  as  lenses  (fig.  699),  focusing  the 
light  upon  the  lower  side  of  the  cell, 
so  that  a  spot  in  the  center  is  much 
more  brightly  illuminated  when  the 
light  strikes  at  right  angles.  The 
position  of  this  area  is  shifted  when 
the  leaf  blade  is  oblique  to  the  rays. 
Correspondingly,  it  is  assumed  that 
the  protoplast  is  excited  when  the 
bright  spot  rests  on  any  but  the  central  area.  There  is  no  doubt  that  the  structures  de- 
scribed concentrate  the  light,  for  that  ran  be  shown  photographically  ;  but  there  are 
sensitive  blades  in  which  domed  epidermal  cells  are  wanting,  and  experiments  do 
not  yet  unequivocally  sustain  the  assumed  distribution  of  irritability.  The  per- 
ceptive organs  of  leaves  have  not  been  located  other  than  by  this  still  doubtful 
hypothesis. 

(7)    Other  tropisms  with  radiant  energy 

Electrotropism. — Currents  of  electricity  passing  through  the  medium  in  which 
plants  are  growing,  and  presumably  through  the  organs  themselves,  evoke  various 
curvatures  according  to  the  density  of  the  <  urrents  used.  By  nature  roots  lend  them- 
selves especially  well  to  experiment.     Some  of  these  responses,  and  possibly  all  of 

them,  are  due  to  one  sided  injury  of  the  roots.     The  effects  appear  to  be  due  to  ele<  - 

trolysis  of  the  solutions  used  ;  but  whether  by  the  dire,  t  ai  tion  of  the  ions  outside 
or  by  the  withdrawal  of  ions  from  the  protoplasl  is  not  certain.  Electrotropism  or 
galvanotropism  may  therefore  be  hardly  more  than  a  spe<  ial  form  of  chemotropism. 

It  does  not  seem  likely  that  such  stimuli  ad  to  any  important  extent  in  nature. 
The  more  important  effects  of  galvanic  and  static  currents  upon  development  have 
already  been  dest  ribed    see  p.  438). 

Thermotropism. — Thermotropism    is    also    of    little    importance.      Both    roots 

and  stems  of  particular  plants  turn  toward  or  away  from  a  bku  lined  plate  radiating 
heat,  according  to  the  temperature.  In  a  similar  way  mots  growing  in  sawdust 
will  grow  toward  or  away  from  a  source  of  conducted  heat.  Wither  form  of 
n-  it  tion  can  be  of  nun  h  importance  in  nature. 

The  same  may  be  said  of  reactions  to  radium  and  its  sails,  as  well  as  those  to 
X-rays.    The  in  jurious  effects  of  these  .ire  more  pronounced  than  the  tropisms. 


480  PHYSIOLOGY 

8.   THE    DEATH    OF    PLANTS 

The  cycle  ends.  —  From  the  foregoing  it  has  become  evident  that  the 
growth  and  development  of  plants  does  not  proceed  uniformly,  but  that  it 
is  profoundly  influenced  —  one  may  even  say  controlled  —  by  external 
conditions;  and  since  many  of  these  external  conditions  evince  a  de- 
cided periodicity,  growth  and  development  exhibit  a  corresponding 
periodicity.  But  it  has  also  become  apparent  that  growth  and  develop- 
ment are  likewise  affected,  and  in  many  particulars  as  profoundly 
affected  or  controlled,  by  factors  that  are  wholly  internal,  so  far  as  is 
known  at  present.  It  is  found,  further,  that  these  factors  may  give  rise  to 
periodicity  in  growth  and  development;  for,  however  uniform  the  exter- 
nal conditions  may  be,  neither  proceeds  uniformly.  In  nothing  is  this 
more  impressively  shown  than  in  the  fact  that  the  cycle  of  development, 
in  spite  of  all  that  can  be  done,  sooner  or  later  comes  to  an  end,  and  the 
plant  perishes,  leaving  behind  comparatively  few  living  cells,  if  indeed  it 
leaves  any,  out  of  the  unnumbered  millions  that  may  have  constituted 
its  body. 

No  inherent  reason  for  death.  —  There  does  not  seem  to  be  any  in- 
herent reason  why  a  plant  should  die.  The  material  of  which  it  is  com- 
posed is  all  the  while  undergoing  decomposition  and  repair.  In  a  per- 
ennial plant,  like  a  tree,  the  tissues  in  great  part  are  renewed  annually, 
so  that  though  the  living  and  the  dead  stand  together  as  a  sort  of  unity, 
which  may  have  occupied  the  place  for  centuries,  the  oldest  of  the  living 
parts  is  only  a  minute  fraction  of  these  centuries  old.  In  such  a  plant, 
however,  it  becomes  increasingly  difficult  to  supply  the  extremities  with 
the  needful  materials,  because  they  are  steadily  becoming  separated  by 
greater  and  greater  distances.  The  leaves  are  yearly  further  from  the 
ports  of  entry  for  water,  and  the  roots  are  yearly  further  from  the  source 
of  food.  With  expanse  of  branching,  mechanical  overthrow  threatens 
more  and  more.  Thus  the  physical  conditions  are  steadily  becoming 
more  severe,  and  it  is  easy  to  imagine  why  the  plant  must  finally  suc- 
cumb. Yet  the  long  persistence,  even  after  it  has  become  evident  that 
a  tree  has  reached  the  practical  limit  of  growth,  shows  that  there  is 
nothing  in  the  living  parts  themselves  which  determines  the  end;  and 
still  more  is  this  shown  by  the  fact  that  cuttings  may  be  taken  from  an 
old  tree  and  successfully  started  upon  a  new  cycle  which  may  be  as  long 
as  the  parent's.  Thus,  the  Washington  elm  at  Cambridge  has  been 
struggling  against  adversity  for  more  than  a  quarter  of  a  century,  slowly 


GROWTH    AND    MOVEMEN1  481 

succumbing  in  a  losing  fight;  but  a  cutting  from  it  is  now  a  thrifty,  well- 
grown  tree  on  the  Boston  Common. 

Reproduction.  —  In  the  smaller  plants  the  inception  of  unfavorable- 
conditions  is  often  a  signal  for  the  gathering  together  of  all  the  living 
material  into  a  form  that  can  endure  adversity,  as  with  the  encystment 
in  lai  teria,  fungi,  and  algae.  Under  these  circumstances  also  the  pro- 
toplasm is  divided  into  several  or  many  parts,  each  appropriately  pro- 
tected; thus  multiplication  becomes  possible  if  more  than  one  part  es- 
capes injury  and  finds  suitable  conditions  again  for  development  (see 
Botrydium,  p.  33,  and  many  other  illustrations  in  Part  I).  This  simple 
situation  has  been  worked  out,  in  the  higher  plants,  into  elaborate  mech- 
anisms of  reproduction,  which  are  now  not  always  obviously  related  to 
the  inception  of  unfavorable  conditions.  Yet  methods  of  cultivation  in- 
dicate that  the  formation  of  spores,  even  in  the  seed  plants,  in  which 
naturally  it  often  far  precedes  the  period  of  flowering,  may  be  initiated 
by  conditions  unfavorable  for  vegetative  growth.  Until  these  conditions 
can  be  more  exactly  designated  and  analyzed,  it  is  unprofitable  to  con- 
sider them  more  in  detail.  At  present,  then,  all  that  can  be  said  is  that 
unfavorable  conditions  bring  about  a  redistribution  of  the  living  mate- 
rial, of  which  as  much  as  possible  resists  and  persists.  Thus,  since  the 
beginning  of  things,  we  assume,  there  has  been  an  unbroken  chain  of 
living  matter,  shaping  itself  for  a  time  into  organisms  more  or  less  com- 
plex, and  then  retiring  into  the  simplest  and  least  exposed  forms,  to  begin 
another  cycle  of  development  when  the  conjunction  of  internal  and 
external  forces  permitted. 

What  is  death?  —  The  abandonment  by  the  living  protoplasm  of  a 
body  previously  constructed,  or  the  destruction  of  the  protoplasm  wholly 
or  in  great  part,  is  what  is  usually  meant  by  the  death  of  a  plant.  Since 
plants  conspicuously  lack  individuality  whenever  they  become  more 
complex  than  a  single  cell,  the  severance  of  a  plant,  even  the  highest, 
into  two  or  more  parts  may  not  bring  death,  as  it  does  to  so  many  of  the 
higher  animals,  but  rather  renewed  vigor.  Correspondingly,  the  death 
«  I  even  a  large  part  of  the  body  does  not  necessarily  bring  death  to  the 
whole,  but  often  likewise  renewed  vigor  to  the  parts  that  persist. 

Local  and  general  death.  —  Extensive  local  death,  as  this  may  be  called 
for  convenience,  i--  possible  in  plants  without  the  serious  consequences 
that  follow  in  the-  higher  animals,  fir~i  because  plants  have  so  little  spe- 
cialization <>f  organs  and  so  many  of  the  same  kind ;  second,  because  they 
have  no  circulatory  system  that  might  rapidly  distribute   to  other  parts 

C.  B.  Sc  C.  BOTANY  —  }I 


482  PHYSIOLOGY 

deleterious  substances  arising  in  the  dead  region,  and  so  cause  their 
injury  or  death;  and  third,  because  they  have  no  nervous  system,  (tut- 
ting into  quick  communication  sound  distant  organs  with  hurtful  stimuli 
from  the  dead  ones.  Yet  these  differences,  on  the  surface  so  marked, 
are  in  reality  not  fundamental,  for  what  is  general  death  in  the  animal 
is  in  reality  only  an  extension  of  local  death  to  the  several  tissues  and 
organs  more  rapidly  than  in  plants.  But  each  part  dies  at  its  own  rate 
and  only  because  the  interruption  of  the  activity  of  one  organ  has  created 
conditions  unfavorable  to  the  other. 

Irreversible  reaction.  —  The  phenomena  of  death  are  not  easily  de- 
scribed. Certain  changes  in  the  appearance  of  the  cytoplasm  are  visible 
under  the  microscope  (such  as  are  familiar  in  fixed  cells  and  are  too  com- 
monly thought  of  as  the  normal  appearance  of  cytoplasm),  chiefly  ag- 
gregation and  vacuolation;  but  the  significance  of  these  is  not  known. 
Alteration  in  the  chemical  processes  and  different  behavior,  especially 
permanent  insensitiveness  to  external  stimuli,  are  the  most  important 
marks  of  death.  During  life  the  protoplasm  is  constantly  adjusting  itself 
to  new  conditions,  each  response  suited  to  the  stimulus,  whether  in  a 
favorable  or  unfavorable  direction.  These  responses  of  normal  life 
are  assumed  to  be  reversible,  as  are  many  chemical  reactions.  But 
when  the  responses  to  severe  stimuli  become  irreversible  in  too  great 
measure,  the  possibility  of  readjustment  to  new  stimuli  is  past ;  this  con- 
dition is  death. 

Diseases.  —  Plants  are  often  killed  by  diseases  which  may  arise  from 
the  disturbance  of  function  wrought  by  external  agents,  such  as  the  ele- 
ments of  climate,  the  solutes  of  the  soil,  gases  in  the  air,  etc.  Or  disease 
may  be  due  to  the  invasion  of  the  body  by  parasites,  which  rob  the  host 
of  food,  interfere  with  its  water  supply,  or  upset  some  necessary  function. 
A  study  of  diseases  forms  a  great  field  in  itself,  plant  pathology,  under 
which  name  therapeutics,  the  study  and  application  of  remedial  measures, 
is  also  usually  comprehended.  It  is  one  of  the  divisions  of  botany  which 
is  of  great  economic  importance,  and  one  whose  study  has  reached  its 
highest  level  in  this  country,  where  the  remedial  and  preventive  meas- 
ures devised  save  annually  many  millions  of  dollars.  The  knowledge 
of  infectious  diseases  has  been  most  extensively  developed,  but  therein 
a  great  field  for  investigation  still  lies  open,  and  a  still  greater  one  in  the 
more  difficult  study  of  functional  disorders. 

Mechanical  injury.  —  Mechanical  injuries  often  lead  to  death,  es- 
pecially because  they  expose  the  plant  to  infection  by  bacteria  and  fungi. 


GROWTH    AND    MOVEMENT  483 

Unwise  pruning  <>f  trees  in  our  cities,  much  more  the  heedless  hacking 
at  the  hands  of  linemen  stringing  telegraph  and  telephone  wires,  and  the 
gnawing  by  horses  carelessly  bitched  t<>  the  trees,  frequently  open  the 

way  for  infection  by  some  deadly  parasite.  Ice  storms,  bail,  wind-,  and 
lightning  all  contribute  to  serious  mechanical  injuries  at  times,  whose 
direct  effects  are  less  to  be  feared  than  the  indiret  t. 

Heat  and  cold.  —  High  temperature  is  a  fruitful  cause  of  local  death, 
for  this  is  often  associated  with  a  deficiency  in  the  water  supply.  There 
has  been  recognized  a  falling  of  the  leaves,  espei  ially  of  trees,  in  mid- 
summer, which  is  due  to  the  heat,  and  may  amount  to  a  large  per  cent  of 
the  total  foliage.  The  older  leaves,  and  those  least  favorably  situated 
for  receiving  sufficient  water  (the  latter  are  at  the  same  time  most  ex- 
posed to  the  direct  rays  of  the  sun)  are  the  ones  that  suffer  most.  Low 
temperatures  kill  tender  plants  by  direct  injury  to  the  protoplasts,  t  ven 
before  the  freezing  point  is  reached.  Others  are  killed  only  by  the  freez- 
ing itself,  probably  because  this  withdraws  water  from  the  protoplast 
and  vacuoles,  thus  concentrating  the  solutions,  perhaps  to  a  point  where 
certain  solutes  may  become  poisonous.  There  are  many  plants,  how- 
ever, which  are  able  to  withstand  freezing,  and  on  gradual  thawing 
the  water  is  taken  back  into  the  protoplast  again.  All  the  trees  and 
shrubs  and  the  persistent  parts  of  herbaceous  perennials  are  liable  to  be 
solidly  frozen,  often  more  than  once,  in  the  winters  of  the  northern 
states  and  Canada,  but  they  usually  bear  this  unharmed,  though  the  trees 
then  have  almost  a  maximum  water  content.  The  most  serious  danger 
in  the  northern  winters,  especially  to  the  evergreens,  is  that  during  a 
warm  period  the  evaporation  will  surpass  the  income  from  the  shaded  and 
frozen  soil. 

Temperature  and  water.  —  In  general  the  proportion  of  water  present 
determines  the  resistance  to  injury  by  low  and  high  temperatures,  other 
things  being  equal.  Thus  air-dry  seeds  withstand  the  lowest  tempera- 
ture yet  tried,  that  of  liquid  hydrogen  (—  25o°(\),'  and  germinate  freely 
when  planted;  while  the  same  seeds,  if  soaked  in  water  until  swollen,  will 
be  killed  at  a  very  much  higher  temperature.  In  like  manner  tempera- 
tures short  of  absolute  charring  an'  borne  by  dry  seeds,  while  a  few  min- 
utes' exposure  at  700  C.  will  kill  soaked  ones.  Similarly,  plants  of  firm 
texture  and  little  sap  withstand  unfavorable  temperatures  better  than 
watery  ones. 

1  Doubtless  they  will  endure  the  temperature  of  liquid  helium  (probably  within  five 
or  six  degrees  of  the  absolute  zero,  —  2730)  if  enough  is  ever  obtained  for  such  a  test. 


484  PHYSIOLOGY 

Poisons.  —  Various  substances,  comprehensively  known  as  poisons, 
kill  the  protoplasts,  when  their  concentration  is  sufficient.  At  lower  con- 
centrations many  of  the  very  same  substances  accelerate  growth  or  develop- 
ment or  special  functions.  The  action  of  these  substances  may  depend 
upon  their  dissociation  in  solution  into  ions,  if  they  are  electrolytes,  or 
upon  the  molecules  themselves,  or  both.  Some  act  by  coagulating  the 
protoplasm  and  others  induce  changes  of  a  different  sort,  not  accurately 
known.  Ionic  hydrogen,  silver,  copper,  and  mercury  are  remarkably 
injurious.  A  solution  of  only  one  part  per  million  of  a  silver  salt  is 
quickly  fatal  to  the  roots  of  lupines,  and  still  less  of  mercury  kills. 
Some  very  important  economic  measures  depend  upon  the  extreme 
sensitiveness  of  protoplasm  to  such  substances.  For  microscopic  study 
it  makes  possible  the  almost  instant  killing  of  the  protoplasts,  and  by 
combining  a  fixing  with  the  killing  agent,  the  preserving  of  the  protoplast 
in  a  form  which  approaches  closely  the  condition  in  life ;  so  far,  at  least, 
as  can  be  judged  from  what  can  be  seen  of  minute  structures  in  the  living 
condition.  Further,  the  poisonous  nature  of  such  substances  makes  it 
possible  to  employ  them  against  the  agents  of  infectious  diseases,  par- 
ticularly those  that  grow  on  the  surface  of  the  host.  The  poisons  act 
at  lower  dilutions  upon  the  parasite,  because  its  protoplasm  is  more  ac- 
cessible than  that  of  the  host,  whose  epidermis  prevents  injury  in  great 
measure.  The  usual  form  in  which  they  are  employed  is  in  solution, 
which  can  be  sprayed  at  appropriate  times  over  the  host.  Many  most 
destructive  diseases  are  thus  held  in  check.  Where  a  disease  is  trans- 
mitted with  the  seed,  they  may  be  disinfected  by  short  soaking  in  a 
suitable  solution,  without  materially  injuring  their  germinative  power. 
The  modern  methods  of  antiseptic  surgery,  personal  and  municipal 
hygiene,  and  the  treatment  of  infectious  diseases  rest  essentially  upon 
like  principles,  for  in  nearly  all  these  cases  the  organisms  to  be  com- 
batted  are  plants.  ^*   *vj 

The  death  of  plants  appropriately  terminates#a|di9<^ssion  of  their 
behavior.  ^&r    ~C* 


INDEX 


[Figures  in  italics  indicate  pages  upon  which  illustrations  occur.] 


Abies,  resin  gland,  340. 

Abietineae,  219- 

Absorption  bands,  369;   spectrum,  368,  369. 

Abstraction,  of  spores,  65. 

Acetic  fermentation,  411. 

Achene,  282. 

Acids,  organic,  414. 

Acorus,  vascular  bundle,  246. 

Acrocarpae,  i.m. 

Acrogynae,  103. 

Acropctal  succession,  in  flowers,  256. 

Actinomorphic  flowers,  256. 

Adder's  tongue,  149. 

Adhesion,  324. 

Adiantum,  stem  section,  160. 

Aecidiospores,  83. 

Aecidium,  83,  84. 

Aerating  system,  318. 

Aerotaxy,  449. 

Aerotropism,  474. 

Aestivation,  282. 

Art  hi  ilium,  3. 

Agaricaceae,  88. 

Agaricales,  87. 

Agaricus,  89. 

Albizzia,  leaf  movements,  456. 

Albugo,  65,  66. 

Alcoholic  fermentation,  409. 

Aleurone  grains,  3Q2. 

Algae,  14. 

Alkaloids,  392,  415. 

Allium,  absorption  spectra,  369. 

Alternation  of  generations,  Coleochaete,  31  ; 

Polysiphonia,     60;      rusts,     84;      higher 

plant:,  92. 
Ament,  279. 
Amides,  s»o,  392. 
Amphigastria,  104. 
Amphithecium,  95,  97,  99,  108,  113,  115,  11S, 

110. 

Am  phi  vasal  bundles,  245,  246. 

Amylase,  400. 

Anabaena,  9. 

Anabolism,  402. 

Vnacrogynae,  101  ;  conclusions,  10?. 

Anaptychia,  80. 

Anatropous  ovules,  261. 


Andreaeales,  114. 

Androspores,  30. 

Aneimia,  sporangium,  137. 

Aneimites,  183. 

Aneura,  101. 

Angiosperms,  180,  238;  classification,  276. 

Anisocarpic  flowers,  281. 

Annual  thickening,  346. 

Annular  vessels,  2  / ,'. 

Annulus,  mushrooms,  89;  ferns,  155,  156. 
157,  163,  164,  165,  352. 

Anther,  256. 

Antheridium,  green  algae,  27,  30,  31,  32,  36, 
42,  43;  Fucus,  50;  Nemalion,  56;  Poly- 
siphonia, so;  fungi,  64,  66,  73,  75,  78; 
liverworts,  92,  Q4,  93,  99,  102,  104,  103, 
106,  107  ;  mosses,  112,  115,  117;  lycopods, 
127,  120,  133,  140;  equisetums,  148; 
ophioglossums,  154;  ferns,  166,  167; 
water  ferns,  174,  175,  178. 

Antherozoid,  17,  56. 

Anthoceros,  106,  107,  108,  ioq. 

Anthocerotales,  100;  conclusions,  109. 

Antipodal  cells,  265. 

Apical  cell,  42,  46,  98. 

Aplanospores,  34. 

Aplastic  products,  412. 

Apocarpous  flowers,  279. 

Apogamy,  Urns,  169;  angiosperms,  275. 

Apogeotropism,  460. 

Apophysis,  119,  120. 

Apospory,  ferns,  169. 

Apostrophe,  450. 

Apothecium,  71,  72,  78,  79,  80. 

Araks,  277. 

Araucarineae,  220. 

Archegonium,  92;  liverworts,  94,  06,  09, 
103,  104,  107;  mosses,  112,  r/j,  115,  •  •  7 . 
1X8;  lycopods.  128,  ;,•«',  [36,  / 37,  140; 
equisetums,     [48;     ophioglossums,     153, 

131;  ferns,  107,  168;  water  ferns,  175, 
I7q;  gymnosperms,  197,  ig8,  IffQ,  205, 
210,  211.  214,  J/5,  216,  223,  233;  complex, 
223 ;  jacket,  198. 

Archicarp,  79. 

Archichlamydeae,  239;  classification,  278. 

Arthrospores,  g. 


INDEX 


Ascobolus,  73. 

Ascocarp,  70,  71,  72,  73,  75,  76. 

Ascogenous  hyphae,  72,  73. 

Ascogonium,  73. 

Ascomycetes,  70. 

Ascospores,  70,  72,  73,  76,  77. 

Ascus,  70,  72,  73,  76,  77,  78,  80. 

Ash,  412,  415. 

Aspergillus,  74. 

Aspidium,  habit  and  sporangia,  163;  gamc- 

tophyte,  166. 
Assimilation,  364,  401. 
Atrichum,  leaf  cells,  450. 
Atropin,  415. 
Auriculariales,  86. 
Autobasidiomycetes,  86. 
Autonomic  movements,  453. 
Autotrophic  plants,  362,  380. 
Auxiliary  cells,  Polysiphonia,  sg,  60. 
Auxospores,  53. 
Azolla,  171,  172,  173,  174,  175. 

Bacillus,  11. 

Bacteria,  10,  //;  aerobic,  13  ;  anaerobic,  13  ; 

iron,    14 ;    nitrifying,    13 ;    nitrogen,    13 ; 

pathogenic,  13  ;  saprophytic,  13  ;  sulphur, 

14. 
Bark,  loss  of,  355. 
Basidiomycetes,  80. 
Basidiospore,  80,  83,  8g. 
Basidium,  80,  82,  S3,  8g. 
Batrachospermum,  57. 
Bennett  itales,  185. 
Bennettites,  strobilus  and  seed,  i8g. 
Bilabiate  flowers,  282. 
Biophores,  291. 

Biophytum,  records  of  responses,  42Q. 
Black  fungi,  76. 
Black  knot,  76. 
Black  mold,  67. 
Bleeding,  332,  334. 
Blepharoplast,  200,  211. 
Blue-green  algae,  4. 
Blue  mold,  74. 
Body    cell,    igg,    200,   211,   215,  217,  224, 

235- 
Bog  mosses,  no. 
Boletus,  88. 
Botrychium,  149 ;   habit,  150;    gametophyte 

and  archegonium,  134. 
Botrydium,  33,  34. 
Bowenia,  102. 

Box  elder,  section  of  stem,  245. 
Branches,  fall  of,  355  ;  origin  of,  419. 
Brown  algae,  44. 
Brucin,  415. 
Bryales,  115. 
Bryophytes,  92. 


Bulbochaete,  30,  31. 
Butyric  fermentation,  411. 

Caffein,  415. 

Callus,  419. 

Calymmatothcca,  184. 

Calyptra,  95,  07,  100,  113,  118. 

Calyptrogen,  247. 

Calyx,  252. 

Cambium,  150,  192,  243,  24g. 

Campanales,  282. 

Campy  lot  ropous  ovule,  261,  262. 

Canna,  megasporangium,  263. 

Capillarity,  and  ascent  of  water,  349. 

Capillitium,  puffballs,  90  ;  slime  molds,  3. 

Capsella,  embryo,  272,  273.  * 

Capsule,  liverworts,  100,  103,  105,  108,  iog; 

mosses,    113,    114,    115,    118,    iiq,    120; 

ferns,  164. 
Carbohydrates,  358,  374. 
Carbon  assimilation,  363. 
Carbon  dioxid,  admission  of,  365 ;    as  raw 

material,  364. 
Carnivorous  plants,  386. 
Carotin,  367,  368. 
Carpel,  260. 

Carpogonium,  56,  57,  59,  60. 
Carpospore,  56,  57,  sg,  60. 
Catabolism,  402. 
Catalysis,  399. 
Catalyst,  399. 
Catkin,  279. 
Cell,  organs,  297;   role  of  living,  351  ;    wall, 

297,  298,  306. 
Cellulose,  299,  359;   "reserve,"  390. 
Central  body,  blue-green  algae,  5. 
Central  cylinder,  124. 
Centripetal  succession,  flowers,  256. 
Ceratozamia,  megasporophyll,  197. 
Chaetophora,  26. 
Chalaza,  266,  269. 
Chalazogamy,  268. 
Chantransia,  58. 
Chara,  apical  cell,  42;  habit,  41;  sex  organs, 

43- 
Charales,  41. 
Chemical  stimuli,  438. 
Chemotaxy,  447. 
Chemotropism,    473 ;     fungi,    473 ;     pollen 

tubes,  474. 
Chlamydomonas,  15. 
Chlamydospore,  81. 
Chlorenchyma,  366. 
Chlorophyceae,  15. 
Chlorophyll,  2,  367. 
Chlorophyllin,  367. 
Chloroplast,  297,  366,  376,  450. 
Chlorovaporization,  330. 


B 


i.\i)i;x 


Chromosomes,  .52,  51,  60.  92.  170,  275. 
Chytridiales,  63. 

Chy Iridium,  <$J. 

Cilia.  445  ;   in  action,  7V<5. 

Cim  bonin,  415- 

('initiate  vernation,  159,  163,  176. 

Cilrii  add,   )i  1 

Citrus,  oil  receptacle,  340  ■ 

Cladophora,  26,  27  ;  walls,  308. 

Cladosiphonic,  159. 

Clavariales,  87. 

Cleistocarpac,  1  jo. 

Cleistothedum,  74,  75. 

( llinostat,  462. 

I  lo  terium,  37,  38. 

(  lull  mosses,  122. 

Clustercup,  83,  84. 

Cocain,  415. 

Coccus,  11. 

Codein,  415. 

Codonotheca,  184. 

Coenocytes,  26,  27,  33, 34, 35, 36, 62. 

Cohesion  theory,  351. 

Cold,  cause  of  death,  483. 

Coleochaete,  31,  32. 

Coleoptile,  and  starch  grains,  465. 

Colony,  blue-green  algae,  6  ;  green  algae,  1 7, 

21. 
Columella,  Anthoceros,  108, 100;  black  mold, 

67,  68;  mosses,  113,  118, 119, 120. 
Companion  cells,  24 3;  role,  394. 
Compass  plants,  478. 
Concentration,  304. 
Concentric  bundles,  125, 161. 
Conceptade.jo. 

Conducting  system,  393  ;  origin,  341. 
Conducting  tissue,  in  style,  260. 
Confervales,  24  ;   conclusions,  a- 
Conidia,  63,  74,  75. 
Coniferales,  212. 
Conjugates,  37  ;  conclusions,  40. 
Conjugation,  16,  38,  39,  40. 
Contact  movements,  454. 
Coprinus,  88,  89. 
Coral  fungi,  87. 
Corallina,  55. 

Cordaitales,  203,  204,  -'"5,  206. 
Cork,  cambium,  240;  cells,  240;  role,  318. 
( lorn,  set  tion  of  stem,  245. 
Cornaceae,  280. 
Corn  smut,  81. 
Corolla,  252,  253. 
Correlations,  441. 
Cortex,  angiosperms,  239,  240,  242;  gymno- 

sperms.  192,  194,  2x9;   kelps,    48;    pteri- 

dophytes,  124,  X2$,  145,  139,  /e><>,  iru,  162. 
Crossing  of  |H>llen,  268. 
Crossotheca,  184. 


Cryptogams,  180. 

Cup  fungi,  71. 

Cupressineae,  220. 

Cupules,  98,  99. 

Curvatures,  growth,  458 ;   nastic,  442. 

Cuscuta,  haustorium,  382. 

Cutin,  299. 

Cutleriaceae,  49. 

Cyanophyceae,  4. 

Cyatheaceae,  156. 

Cycadales,  190. 

Cycadella,  185. 

Cycadeoidea,  habit,  183;  strobilus,  186,  187, 

iSS;  synangia,  189. 
Cycadofilicales,  181. 
Cycadofilices,  181. 
Cycas,    habit,    190;    microsporophyll,    105; 

megasporophyll,  197;   male  gametophyte, 

199;  embryo,  202. 
Cystocarp,  56,  57,  59. 
Cytase,  400. 
Cytoplasm,  2,  297. 

Dacromycetales,  86. 

Dacrydium,  male  gametophyte,  217. 

Daily  period,  436. 

Death,  480. 

Decay,  385. 

Deformities,  440. 

Dendroceros,  106. 

Dermatogen,  239,  240,  247,  465. 

Desmidiaceae,  37. 

Desmids,  37. 

Desmodium,  leaflets,  453. 

Determinants,  291. 

Development,  417. 

Deztrinase,  400. 

Diageotropism,  467. 

Diaphragm,  water  ferns,  175. 

Diastase,  399. 

Diatomin,  53. 

Diatoms,  52,  34. 

Dichotomous,  branching,  49;  venation,    159, 

163,  164. 
Dicotyledons,      238;       classification,      279; 

embryo,  271  ;   vascular  system,  243. 
Dictyotales,  55. 

I  diffusion,  302,  304,  393  ;  rate,  304. 
Digestion,  307  ;  extracellular,  398. 
Dioedous,  30,  105. 
I  lioedsm,  30. 
Dionaea,  386,  387. 
Dioon,  embryo  sac.  roo;  habit,  191;  ovule, 

198;  staminate  strobilus,  195. 
Dioscorea,  ocella,  479. 
Discomycetes,  72. 
Disease,  482. 
Disk  flowers,  282. 


INDEX 


Dodders,  472. 
1  >orsiventrality,  437. 
I  (orj  cordaites,  204. 
Dotted  ducts,  241,  243. 
Double  fertilization,  269. 
Downy  mildews,  65. 
Drosera,  387,  454. 

Ear  fungi,  86. 

Earth  star,  90. 

Ebenales,  281. 

Ecology,  295. 

Ectocarpus,  45. 

Ectoplast,  306. 

Efficiency,  371. 

Egg,  thallophytes,  17,  18,  19,  28,  30,  32,  36, 
51,  64;  bryophytes,  96,  107,  118;  pteri- 
dophytes,  130,  137,  140,  154,  168,  179; 
spermatophytes,  198,  216,  217,  226,  265, 
266,  269. 

Egg  apparatus,  265. 

Eichhomia,  megaspore  tetrad,  26 3. 

Elaterophore,  103. 

Elaters,  100. 

Electric  waves,  438. 

Electrotropism,  479. 

Eligulatae,  132. 

Embryo,  angiosperms,  271,  272,  273,  274, 
275;  Botrychium,  153,  154;  Equisetum, 
149;  ferns,  168,  169;  gymnosperms,  189, 
200,  201,  202,  211,  212,  218,  225,  226,  227, 
236,  237;  Isoetes,  141;  Lycopodium,  130, 
131;  Selaginella,  137. 

Embryo  sac,  197, 198, 199,  234,  261,  264,  265, 
266. 

Endarch,  157. 

Endodermis,  240. 

Endogenous,  250  ;  root  branches,  248. 

Endo^>erm,  202,  211,  223,  270. 

Endothecium,  anthers,  257,  238,  259;  bryo- 
phytes, 95,  97,  99,  108,  113,  115.  n8,  Tig. 

Energy,  368  ;   absorbed,  370. 

Entomophthorales,  68. 

Environment,  284. 

Enzymes,  3g  ;  carbohydrate,  399  ;  fat,  400  ; 
glucoside,  400  ;  protein,  401. 

Ephedra,  archegonia,  233;  embryo,  236, 
237;   habit,  228;   male  gametophyte,  233. 

Epidermis,  angiosperms,  239,  240,  242,  248, 
250,  251,  319;  bryophytes,  g4,  98,  109, 
118,  120;  pteridophytes,  145,  160,  161. 

Epigyny,  255. 

Epinasty,  442. 

Epistrophe,  450. 

Epithem,  332. 

Equilibrium,  position  of,  463. 

Equisetales,  143  ;   conclusions,  149. 

Equisetum,  143  ;  antheridium,  148;  embryo, 


149;     gametophyte,     147;     habit,     144; 

sporangium,  146;  stem  section,  145. 
Ergot  fungus,  77. 
Ericales,  281. 
Erysiphaceae,  75. 
Erysiphe,  haustorium,  381. 
Etiolin,  367. 
Eudorina,  17,  18. 
Euglcna,  20. 
Eumycetes,  62. 
Eurotium,  74. 
Eusporangiates,  126. 
Evaporation,  323. 
Evolution,  283. 
Exarch,  157. 
Excitability,  434. 
Exine,  147,  258. 
Exoascus,  71. 
Exobasidiales,  86. 

Exogenous,  origin  of  branches,  419. 
Exudation,  332,  333;  cause,  335. 

Fagus,  mycorhiza,  382. 
Farinales,  278. 
Fat  enzymes,  400. 
Fatigue,  432. 
Fats,  360,  391. 

Fermentation,  385,  409;    acetic,  411;    alco- 
holic, 409 ;  butyric,  411;  lactic,  410. 
Ferns,  155. 
Fertilization,   gymnosperms,    201,   211,    215, 

217,  225,  226,  235,  268,  269;  mosses,  116; 

pteridophytes,  136,  16S ;  thallophytes,  18, 

28,  66. 
Ficus,  leaf  skeleton,  343. 
Filament,  of  anther,  256,  257. 
Filicales,  155. 

Filicineae,  155;   conclusions,  170. 
Filmy  ferns,  155. 
Flagella,  445. 
Flagellates,  20. 
Florideae,  55. 
Flower,  180,  251. 
Flowering  plants,  180. 
Fluorescence,  370. 
Flytrap,  386,  3$7- 
Food,    356 ;     and   growth,    363 ;     source   of 

energy,  363  ;   storage,  388  ;    translocation, 

388. 
Foot,  bryophytes,   wo,    103,   108,  109,   113, 

1 14;    pteridophytes,    130,    131,   137,    141. 

149,  154,  168. 
Formaldehyde,  360. 
Form  and  light,  437. 
Formative  stimuli,  435. 
Fragmentation,  blue-green  algae,  7. 
Friction,  as  stimulus,  470. 
Fronds,  159. 


[NDEX 


Fructose,  3Sg. 
Fruits,  loss  of,  355- 
I  males,  4Q. 

Fucus,  fertilization  and  embryo,  32;    habit, 

to;  sex  organs,  50,  51. 
Fuligo,  aethalium,  3  ;  Plasmodium,  -'. 
I  1111.  linn,  2D7  ;   unit  of,  2g8. 
Fungi,  01  ;  chemotropism  of,  473. 

Calls,    ]Q4,  .(40. 

Gametangium,  43,  46. 

Gamete,  t6,  17,  \6,  40- 

Gametophyte,  32;  thallophytes,  52,  60.  85; 
liverworts,  92,  pj,  07,  101,  103,  10O ; 
mosses,  in,  115;  pteridophytes,  127,  128, 
m,  147,  148,  tS3,  i').S,  166,  167;  female, 
136,  140,  174,  175,   17S,   T7Q,     [96,   /pp,  205, 

210,  211,  214,  2/(5,  223,  232,   234,  264,  26s, 

266;    male,  133,  /^o,   174,    175,  178,  iqq, 

205,  aotf,  210,  215,  217,  223,  224,  235,  267. 
Gases,  diffusion,  302  ;    entry  and  exit,  322  ; 

exclusion,  318  ;   from  shoot,  352. 
« iasteromycetes,  89. 
( ieaster,  go. 
Gemmae,  gS,  104,  117. 
Generative  cell,  iqq,  224,  267. 
< ientianales,  282. 
Geotaxy,  450. 

Geotropism,  4sg  ;  lateral,  467. 
Geranium,  section  of  cortex,  240. 
( lerminal  selection,  2gi. 
Gill,  of  mushrooms,  88,  Sq. 
Gill  fungi,  88. 
Ginkgo,    leaf,    207;     female     gametophyte, 

211;   ovule,  210;   procmbryo,   212;    stro- 

bili,  208, 
Ginkgoales,  207. 
Girdles,  leaf  trace,  193,  10 /. 
Girdling,  347. 
( Hands,  geranium,  337;  form,  338;  Syringa, 

as,-  nectar,  yo;  resin,  ,•/<>. 
( ileba,  ( iasteromyi  otes,  00. 
Gleichenia,  sori,  136;  stele,  i$q. 
Gleicheniaceae,  155. 
Glochidia,  174,  '75- 
<  lloeoi  apsa,  5. 
( lloeothece,  .=;. 
Glucose,  350,  .575- 
Glucoside  enzymes,  400. 
Glumales,  276. 
Glumes,  277. 
Gnetales,  228. 
Gnetum,  embryo,  236;  female  gametophyte, 

234;  ovule,  234;  strobili,  231,  232. 
Gonidia,  ig. 
Gradient,  304. 
(".rand  period,  421,  422. 
( irape  mildew,  00. 


Gravity,  movements,  45s;  nastic  curvatures, 

1 1  |. 
Green  algae,  is- 

Growing  point,  angiosperms,  239,  240,  247. 
( Irowing  regions,  422. 
Growth,  417;    curvatures,  458;    and    food, 

363;    and    light,   435;    movements,    i s 7  : 

rapidity,    424;     and    transpiration 

and  turgor,  510. 
Guard  (ells,  251,  320. 
Gulfweed,  52,  53. 
( iums,  1 1  ,- 
Guttation,    332;     artificial,    353;     in    fungi, 

333  ;  nightly,   , 
Gymnosperms,  180,  181. 

llaustorium,    fungi,    61,    381,    3g8 ;     pollen 

tube,  201. 
Heat,  cause  of  death,  483  ;   from  respiration, 

407. 
Ilelminthostachys,  i4g;   habit,  151. 
Helobiales,  276. 

llelolism,  382. 

Helvellales,  71. 

Hemerocallis,  nectar  gland,  33Q. 

Hemitelia,  si>orangium,  57. 

Hepaticae,  g3. 

Herbarium  mold,  74. 

Heredity,  2g3. 

Heterangium,  182. 

Heterocysts,  7,  8,  q. 

Heterogamy,  17. 

Heterospory,  132,  133,  134. 

Heterotrophic  plants,  362,  380. 

Hippuris,  stem  ti|>,  240,  41S. 

Homospory,  134. 

Hormogonia,  S. 

Horsetails,  143. 

Host,  "i,  381. 

Humidity,  and  transpiration,  329. 

Hybrids,  268,  2g3. 

eae,  88. 
Hydnum,  88. 

Hydrodictyon,  21,  22,  23,  (45. 
Hydropteridineae,  170;   conclusions,  i7g. 
Hydrotropism,  475. 
1  [ymenium,  71. 
I  [ymenogastrales,  w. 
1 1>  menomj  1  etes,  86. 
ll>  menophj  llaceae,  1 55. 
1 1\  menophyllum,  sorus,  136. 
Hyphae,  61. 
Hypogyny,  255. 
Hyponasty,  1 1.-. 
Hypophysis,  271,  273. 

Imbibition,  300. 

[mpatiens,  geotropic  curvature,  461, 


INDEX 


Income,  material,  297. 

Indusium,  156,  157,  163,  165,  172,  175,  177- 

Injury,  440  ;  mechanical,  482. 

Insectivorous  plants,  386. 

Integument,    183,    196,    205,    209,   210,    213, 

214,  230,  232,  233,  261,  280. 
Internodes,  41,  145. 
Intine,  147,  258. 
Inulase,  400. 
Inulin,  391. 
Invertase,  400. 
Involucre,  280. 

Irregularity,  flowers,  256,  278,  280,  282. 
Irritability,  426  ;   loss  of,  434. 
Isocarpic  flowers,  281. 
Isoetes,   embryo,  141;    gametophytes,  140; 

habit,  138;   sporangia,  139. 
Isogamy,  16. 
Isolation,  292. 

Jungermanniales,  101 ;  contrast  with  Mar- 
chantiales,  105. 

Laboulbeniales,  77. 

Lactic  fermentation,  410. 

Lactuca,  root  hairs,  312. 

Lagenostoma,  182. 

Laminariaceae,  47. 

Laminaria,  46. 

Latex  system,  396. 

Leaf,  angiosperms,  250,  251 ;  fall  of,  354 ; 
gaps,  159;  gymnosperms,  100,  101,  102, 
103,  203,  204,  207,  208,  220,  229 ;  liver- 
worts, 101,  104;  mosses,  m,  112,  116; 
pteridophytes,  122,  133,  138,  150,  151, 
158,  163,  164,  171,  176;  traces,  125,  192, 
104. 

Leafy  liverworts,  101. 

Lecithins,  360. 

Leguminosae,  280  ;  relation  to  nitrogen,  379. 

Lenticel,  240,  241. 

Leptosporangiates,  162. 

Lessonia,  48. 

Leucoplast,  380. 

Lichens,  78,  91. 

Life,  408. 

Light,  exposure  to,  370;  and  form,  437; 
and  growth,  435;  and  nastic  curvatures, 
443  ;  photosynthesis,  368;  position,  478 ; 
source  of,  372. 

Ligulatae,  132. 

Ligule,  Lycopodiales,  132,  134,  137,  130,  141. 

Liliales,  278. 

Lily,  anther  section,  2 so;  leaf  epidermis, 
231 ;  leaf  section,  250,  31 9. 

Liquids,  302. 

Liverworts,  93. 

Locomotion,  444. 


Lycoperdales,  00. 

Lycoperdon,  90. 

Lycopodiaceae,  conclusions,  132. 

Lycopodiales,  122. 

Lycopodium,  122  ;  antheridium,  120;  arche- 
gonium,  130;  embryo,  1 31 ;  gametophyte, 
127,  128,  1 20;  habit,  12 3,  134;  sporan- 
gium, 123,  126;  stele,  125. 

Lyginodendron,  stem  section?  182. 

Lygodium,  sporangium,  157. 

Lyngbya,  6. 

Macrocystis,  habit,  47. 

Malic  acid,  414. 

Maltase,  400. 

Manubrium,  43. 

Maple  sap,  334. 

Marattia,  embryo,  169;  habit,  158;  leaflet, 
164. 

Marattiaceae,  155  ;  antheridium,  166 ;  spo- 
rangia, 160. 

Marchantia,  08,  00,  100,  435. 

Marchantiaceae,  97. 

Marchantiales,  93 ;  contrast  with  Junger- 
manniales, 105. 

Marsilea,  female  gametophyte,  170;  habit, 
176;  male  gametophyte,  178;  sporocarp, 
177. 

Marsileaceae,  176. 

Massulae,  water  ferns,  173,  175. 

Material  income,  297. 

Material  outgo,  323. 

Mechanical  stimuli,  439. 

Medulla,  kelps,  48. 

Medullosa,  182. 

Megaceros,  106. 

Megasporangium,  134,  135,  139,  172,  173, 
174,  177,  261,  262,  263. 

Megaspore,  135,  172,  196,  262. 

Megasporocarp,  171,  172. 

Megasporophyll,  135,  107. 

Members,  297. 

Membrane,  cell  wall,  306 ;  cytoplasmic, 
306  ;   impermeable,  305  ;   permeable,  305. 

Mendel's  law,  292. 

Merismopedia,  6. 

Meristem,  133 ;  primary,  419 ;  secondary, 
419. 

Mesarch,  157. 

Mesembryanthemum,  origin  of  lateral  root, 
420. 

Mesocarpaceae,  38. 

Mesophyll,  250. 

Metabolism,  402  ;  destructive,  403. 

Metaxylem,  157,  241. 

Micellae,  300. 

Microcycas,  201. 

Microorganisms,  409. 


[NDEX 


Micropyle,  183,  261. 
Microsphaera,  73,  76. 
Microsporangium,  134,  135,  130,  172,  173, 

'74.  '77.  184,  l89,  -"->.  -'-".  »57i  -' 
Microspore,  133,  175. 
Microsporocarp,  171,  172,  173,  174. 

Mi.  rosporophyu,  135,  795,  214,  220,  257. 

Mi. Ml.-  layers,  anthers,  257. 

Mildews,  75,  76. 

Mimosa,  leaf,  432. 

Miscible,  303. 

Monadelphous  stamens,  255. 

Monoblepharis,  64. 

Monocotyledons,  classification,  276;  em- 
bryo, 273,  274,  275;  vascular  system,  244, 
245,  246. 

Monosiphonous,  algae,  45. 

Moonwort,  i4g,  150. 

Morchella,  71. 

Morel,  71. 

Morphin,  415. 

Morphogeny:  stimuli,  435. 

Morphology,  1,  295. 

Mosses,  no. 

Mother  cell,  127. 

Motor  organs,  451. 

Movement,  417 ;  amoeboid,  444;  autono- 
mic, 453;  of  cell  organs,  430;  ciliary,  445, 
446;  contact,  432,  434;  excretory,  //<,- 
gravity,  455  ;  growth,  457  ;  leaf,  455,  fs6; 
nyctitropic,  436 ;  paratonic,  454 ;  photeo- 
lic,  455,  456;  turgor,  451,  457. 

Mucilage,  blue-green  algae,  6. 

Mucor,  67,  68,  6g,  435. 

Mucorales,  67. 

Muscarin,  415. 

Must  i.  no. 

Mushrooms,  87. 

Mutation,  288. 

Mutualism,  382. 

Mycelium,  61. 

Mycetozoa,  2. 

Mycorhiza,  74,  382,  383. 

Myriophyllum,  stem  section,  320. 

Myxobacteriaceae,  14. 

Narcotin,  415. 

Nastic  movements,  431. 

Nasties,  432. 

Natural  selection,  285. 

Nectary,  339. 

Nemalion,  36. 

Neottia,  mycorhiza,  383. 

Nepenthes,  leaf,  385. 

Nephrodium,  sperm,  444. 

Nereocystis,  47,  48. 

igi,  90. 
Nicotiana,  flower,  253. 


114,  119,  120. 

49;  conclusions,  154. 

^9 ;  habit,  150;  sporangium 


Nicotin,  415. 

Nidulariales,  90. 

Nitella,  42. 

Nitrogen,  source  of,  378. 

Nodes,  41,  145. 

Nostoc,  7,  8. 

Notothylas,  106,  108. 

Nucellus,  183. 

Nucleus,  6,  15,  16,  2Q7. 

Nutation,  423,  424. 

Nutrition,  356. 

Nutritive  mechanism,  in  ovule,  266. 

Oedogonium,  27,  29,  30. 

Oil  receptacle,  340. 

Oils,  essential,  413. 

Ontogeny,  295. 

Oogonium,  27,  28,  30,  31,  36. 

Oomycetes,  62. 

Ooplasm,  66. 

Oosphere,  17. 

Oospore,  18. 

Operculum,  113 

Ophioglossales, 

Ophioglossum,  1 
*J2,  i53- 

Orchidales,  278. 

Organ,  297. 

Organized  bodies,  structures,  300. 

Organogeny,  flowers,  256. 

Orthogenesis,  289. 

Orthotropic  organs,  459. 

Orthotropous  ovules,  261. 

Oscillatoria,  6,  7. 

Osmosis,  302,  305. 

Osmotic  pressure,  309. 

Osmunda.  sporangium,  136;  stele,  162. 

Osmundaceae,  155. 

Outgo,  material,  323. 

Ovary,  260. 

Ovule,  183;  angiosperms,  260,  261;  gym- 
oosperms,  196,  107,  198,  205,  206,  20Q,  210, 
213,  214,  221,  222,  232,  233,  234. 

Oxalic  acid,  414. 

Palisade,  leaf,  250,  251,  31 0. 

Palmales,  277. 

Palmella,  26. 

Pandanates,  276. 

Pandorina,  17. 

Panicum,  coleoptile,  465. 

Panmixia,  29). 

Pappus,  282. 

Parallelotropic  organs,  458,  460. 

Paraphyses,  30.  31,  117. 

Parasite.  6l,  381  ;   injury  by,  383. 

Parasitism.   ;8i. 

Paratonic  movements,  454. 


INDEX 


Parmelia,  79. 

Parthenogenesis,  40,  64,  169,  275. 

Peach  curl,  71. 

Pecopteris,  seeds,  1S3. 

Pediastrum,  21,  22. 

Pelargonium,  capitate  hairs,  337. 

Pallia,  thallus,  iot. 

Pellionia,  starch  grains,  38Q. 

Penicillium,  74. 

Pentacyclic  flowers,  281. 

Peony,  flower,  252. 

Perceptive  region,  430,  463,  465,  477,  479. 

Perianth,  252. 

Periblem,  239,  240,  247. 

Pericentral  cell,  59,  60. 

Pericycle,  241. 

Periderm,  419. 

Peridineae,  54. 

Peridium,  90. 

Perigyny,  255. 

Perinium,  144,  147,  174,  175. 

Periplasm,  66. 

Perisperm,  270. 

Perithecium,  76,  77,  78. 

Peronospora,  67. 

Peronosporales,  65. 

Persistence,  457. 

Petal,  252. 

Petioles,  sensitive,  472. 

Peziza,  72. 

Pezizales,  71. 

Phaeophyceae,  44. 

Phaeosporales,  45. 

Phallales,  91. 

Phanerogams,  180. 

Phaseolus,  in  darkness,  437;  leaf  movements, 
456. 

Phellogen,  240,  419. 

Phloem,  1 24,  345,  394. 

Phosphorus,  source  of,  379. 

Photeolic  movements,  455,  456. 

Photosynthesis,  363 ;  process,  375 ;  prod- 
ucts, 373. 

Phototaxy,  449. 

Phototropism,  475. 

Phycocyanin,  4. 

Phycoerythrin,  55. 

Phycomycetes,  62  ;  conclusions,  69. 

Phycophaein,  44. 

Phycoxanthin,  44. 

Phylloglossum,  131,  132. 

Phyllosiphonic,  159. 

Phylogeny,  295. 

Physcia,  78. 

Physiology,  295. 

Phytophthora,  66. 

Pileus,  87,  88,  89. 

Pilobolus,  68,  333. 


Pilularia,  176. 

Pinaceae,  219. 

Pine,  archegonium,  223;  embryo,  226; 
male  gametophyte,  224;  needle,  220; 
pollen,  221;  pollen  tube,  225;  stem  sec- 
tion, 219;  strobili,  220,  221;  wounded, 
305- 

Pistil,  253,  234,  260. 

Pitcher  plants,  385,  386. 

Pith  rays,  150. 

Pitted  vessels,  241. 

Placenta,  260. 

Plagiotropic  organs,  458,  466,  478. 

Plantaginales,  282. 

Plasmodium,  2. 

Plasmolysis,  309. 

Plasmopara,  66. 

Plectascales,  74. 

Plerome,  239,  240,  247. 

Pleurocarpae,  121. 

Pleurococcus,  20,  21. 

Plowrightia,  76. 

Plum  pockets,  71. 

Poa,  penetrated  by  fungus,  381. 

Podocarpineae,  212. 

Podocarpus,  212;   microsporophylls,  214. 

Poisons,  4S4. 

Polarity,  440. 

Pollen,  199;  chamber,  183,  198,210;  chemo- 
tropism  of,  474;  sac,  260;  tube,  201,  211, 
216,  217,  225,  235,  268,  269. 

Pollination,  268. 

Pollinium,  259. 

Polyembryony,  275. 

Polyhedra,  23. 

Polymorphism,  rusts,  83. 

Polypodiaceae,  156;  antheridium,  167, 
sporangia,  162. 

Polyporaceae,  88. 

Polyporus,  88. 

Polysiphonia,  58,  59,  60. 

Polysiphonous,  algae,  45. 

Polystele,  157. 

Polystichum,  sporangium,  352. 

Pore  fungi,  88. 

Porella,  103,  104,  105. 

Porogamy,  269. 

Portulaca,  photeolic  movements,  455. 

Postelsia,  habit,  48. 

Potamogeton,  escape  of  gas  bubbles,  377. 

Potato  rot,  66. 

Presentation  time,  434,  461. 

Pressure,  atmospheric,  350 ;  barometric, 
329  ;  diffusion,  304  ;  reot,  349. 

Primary  tubercle,  Lycopodium,  127. 

Primulales,  281. 

Procarp,  36,  sg,  60. 

Proembryo,  cycads,  200  201,  202 ;  Ginkgo, 


IX'DKX 


212;    Torreya,    218;     Pinus,    2251    2*6 1 
Ephedra,    2,30",    237;     angiosperms,    --71 

Prog<  otropism,  400. 

Promycelium,  £j. 

Prosenchyma,  1 1  2. 

Protective  tissues,  318. 

Protein  enzymes,  401. 

Proteins,  361,  39]  ;   synthesis  of,  377. 

Prothallial  tubes,  Tumboa,  235. 

Prothallium,  165,  166. 

Protoascales,  70. 

Protobasidiomycetes,  Sr. 

Protococcales,  20  ;  conclusions,  24. 

Protodiscales,  71. 

Protonema,  1 15,  116. 

Protoplast,  7,  207  ;  work  of,  298. 

Protosiphon,  34. 

Protostele,  125,  157,  139. 

Protoxylem,  157.  241. 

Pseudopodium,    slime    molds,    2;     mosses, 

113,  //./.  "S- 
Psilotales,  142. 
Psilotum,  142,  143. 
Pteritlophytes,  122. 
Pteris,  stem  section,  161. 
Pucdnia,  82,  83,  84,  85. 
Puffballs,  00. 
Pulque,  334. 
Pulvinus,  452. 

Purslane,  photeolic  movements,  455. 
Putrefaction,  355,  411. 
Pycnidia,  83. 
I'y<  nidiospores,  83. 
Pyrenoid,  16,  30,  40. 
Pyrenomycetales,  75. 
Pyronema,  72,  73. 

Quillworts,  138. 

Radial  bundles,  248,  24Q. 

Ramentum,  185. 

Ranales,  279. 

Ranunculus,  nectar  gland,  330;  root  section, 

2  n- 

Ray  Bowers,  282. 

Reaction,    mechanism,    431;      modes,    428; 
time.  433. 

Receptat  le,  flower,  253  ;  Man  hantia,  99. 
Red  algae,  54. 
Regular  flowers,  256. 
Reproduction,  481. 
Resins,  413. 

Respiration,    t.\; :     aerobic   and   anaerobic, 
404;     products,    4011;     rule    of    oxygen, 

400. 
Reversible  ai  tion,  109, 
Revolution,  twiners,  40S. 


Rheotropi  n 

Rhipsalis,  chloroplasts,  376. 
Rhizopus,  67. 
Rhodophyceae,  54. 

94,  95,  96,  97- 
Riii  iaceae,  93  ;   con<  lusions,  96. 

I\i.  I  k>l  ,11  [HI-.,   i)  ;. 

Kuiniis,  endosperm  1  ell,  392;  stem  section, 
-'./-'.  346. 

RingleSS  terns,  155. 

Rivularia,  8,  9. 

Rockweed,  10. 

Km., 1.  angiosperm,  247,  248,  249;  branches, 
248;  cap,  246,  j  17.  f6s;  diffusion  from, 
353;  effect  on  soil,  315;  hair>,  J47,  248, 
ji2]  permeable  regions,  ;n  ;  "pressure," 
336;  pt  endophytics,  122,  1.50,  131,  137, 
1  /'■  163,  168;  system,  311  ;  tip,  247. 

Roripa,  rootcap,  if>5- 

Rosales,   j.So. 

Rotation,  ( linostat,  462;  twiner,  468. 

Rubiales,  282. 

Rusts,  82,  83,  84,  85. 

Saccharomycetes,  70. 

Saccharose,  359. 

Sac  fungi,  70. 

Sagittaria,  embryo,  273,  274,  275. 

Salix,  megasporangium,  262. 

Salts,    and    transpiration,   325;    and   water- 

proofing,  ,w~. 
Salvinia,  /,-/. 
Salviniaceae,  171. 
Sa[>  pressure  and  turgor,  311. 
Saprolegnia,  64. 
Saprolegniales,  63. 
Saprophytes,  2,  61,  384. 
Sargassum,  52,  53. 
Sarracenia,  iS'>. 
Scalariform  vessels,  241,  244. 

Scale  mosses,  101. 

Scenedesmus,  21. 

Sceptridium,  149. 

Schizaeaceae,  155. 

S(  hizomyi  etes,  10. 

Schizophyceae,  4. 

Schizophytes,  |. 

Si  itaminales,  278. 

s<  lerodermales,  <.*>. 

Sclerotium,  2,  77. 

Scouring  rushes,  143. 

Si  5  tonem 

Secondary  sylem,  241,  144,  J45. 

Se.  retion,  ,  ,.■.  1 .7  ;  emission,  338. 

Sedum,  stoma, 

Seeds,    r8o,   r*2,   183,  188,  189,  212,  23a; 

loss  of,  355. 
Seed  plants,  1S0. 


INDEX 


Selaginefla,  132  ;  archegonium,  137;  embryo, 
137;  female  gametophyte,  136;  habit, 
133;  sporangia,  134;  spores,  135. 

Selection,  variable,  307. 

Selective  action,  307. 

Sensitive  plants,  429. 

Sepal,  252. 

Seta,  100,  101,  103,  104,  105,  113,  116,  120. 

Shoot,  permeable  regions  of,  311. 

Sieve  plates,  242,  243. 

Sieve  vessels,  242,  243;  role,  394. 

Silphium,  fertilization,  26q;  male  gameto- 
phyte, 267;  microsporangium,  258. 

Siphonales,  33  ;  conclusions,  37. 

Siphonostele,  133 ;  amphiphloic,  157,  160; 
ectophloic,  157,  162. 

Sleep  movements,  455. 

Soil,  312;  capacity  for  water,  313  ;  effect  of 
roots,  315;  water  of,  313. 

Solids,  302. 

Solutes,  303  ;  entry  of,  316 ;  natural,  303. 

Solution,  300,  303. 

Solvent,  303. 

Soredia,  79. 

Sorus,  156, 163, 165. 

Spadix,  277. 

Spathe,  277. 

Spectrum,  absorption,  368,  36Q. 

Sperms,  1 7  ;  thallophy tes,  18,  19,  28,  30,  36, 
43,  50,  52,  56;  bryophytes,  92,  93,  102, 
112,  117;  pteridophytes,  128,  129,  135, 
140,  148,  167,  178,  444;  gymnosperms, 
199,  201,  211. 

Spermatium,  red  algae,  56  ;  rusts,  83. 

Spermatophytes,  180. 

Spermatozoid,  17,  56. 

Spermogonium,  lichens,  79 ;  rusts,  83. 

Sphacelaria,  46. 

Sphaerella,  75. 

Sphaerocarpus,  105. 

Sphaeroplea,  27,  28. 

Sphaerotheca,  75. 

Sphagnales,  no;  conclusions,  114. 

Sphagnum,  antheridia,  112;  archegonia, 
113;  gametophyte,  111;  habit,  112,  113; 
leaf,  in;  sporophytes,  113,  114. 

Sphenophy  Hales,  143. 

Sphenophyllum,  143. 

Spiral  vessels,  241,  243. 

Spirillum,  11. 

Spongy  region,  leaf,  250,  251. 

Sporangiophore,  143,  146,  333. 

Sporangium,  3  ;  thallophytes,  3,  27,  45,  58, 
SO,  67,  68;  pteridophytes,  125,  126,  133, 
134,  130,  142,  143,  144,  146,  152,  153,  156, 
157,  160,  163,  164,  165,  352. 

Spores,  3,  16,  62,  147. 

Sporidia,  83. 


Sporocarp,  171,  172,  173,  174,  176,  177. 
Sporogonium,  95. 

Sporophores,  62. 

Sporophylls,  122. 

Sporophyte,  32  ;  thallophytes,  32,  60,  84 
bryophytes,  92,  95,  97,  99,  100,  lot,  103 
104,  105,  108,  100,  113,  114,  115,  II0»  118, 
iiq,  120;  pteridophytes,  122,  132,  138. 
143,  149,  156,  171,  176 ;  gymnosperms,  181 
185,  191,  203,  207,  213,  220,  229. 

Spirogyra,  30. 

Sprout  chains,  yeast,  70,  71. 

Squirting  fungus,  68. 

Stalk  cell,  200. 

Stamens,  183,  184,  186,  187,  195,  205,  208, 
213,  214,  220,  22i,  230,  231,  252,  253,  256, 
257- 

Staminodia,  281. 

Starch,  358,  375,  389. 

Starch  grain,  38Q. 

Statolith  theory,  464. 

Stegocarpae,  121. 

Stele,  124,  239,  241. 

Stem,  angiosperms,  239. 

Stemonitis,  3. 

Sterigmata,  80,  89. 

Stigeoclonium,  26. 

Stigma,  260. 

Stigmatomyces,  78. 

Stigonema,  10. 

Stimulus,  426 ;  chemical,  438 ;  formative, 
435  ;  mechanical,  439  ;  morphogenic,  435  ; 
tonic,  449. 

Stink  horns,  91. 

Stipe,  mushroom,  87. 

Stomata,  109,  146,  250,  251,  319,  320,  327; 
regulation  of,  327. 

Stomium,  164. 

Stoneworts,  41. 

Storage,  388. 

Straightening,  twiners,  469. 

Strains,  sexual,  68. 

Streaming,  444,  451. 

Strobilus,  122  ;  pteridophytes,  124,  132,  133, 
144;  gymnosperms,  186,  187,  188,  i8q, 
iq2,  193,  iqs,  ig6,  204,  205,  206,  208,  209, 
213,  214,  220,  221,  222,  228,  229,  230,  231, 
232;  theory  of,  123. 

Stroma,  chloroplasts,  367  ;  fungi,  76,  77. 

Style,  260. 

Substratum,  fungi,  61. 

Sugar,  390 ;  cane,  359 ;  fruit,  359 ;  grape, 
359- 

Sulfur,  source  of,  379. 

Summation,  432,  462. 

Sundew,  387,  454. 

Sunflower,  nutation,  424. 

Suspensor,  angiosperms,  271,  272,  273,  274; 


INDEX 


gymnosperm 

Mu... i.  68,  69;    pteridophytes,   130,  / ,'/, 
/  .7- 

Swarm  spores,  r6,  11  >. 

Swelling,  300. 

Swimming  spores,  10. 

Sympetalae,  239;  classification,  280. 

Sympetalous  corolla,  255. 

Sympetaly,  255. 

Symphyogyna,  roi. 

Synangium,  t6i,  r<5  i- 

Synanthales,  277. 

Syncarpous  pistils,  255. 

Syncarpy,  255. 

Synchj  trium,  63. 

Synsepalous  calyx,  255. 

Syringa,  leaf  gland,  338;  leaf  xylem,  343. 

Tannins,  in. 

Tapetum,  126,  153,  164,  174,  177,  257,  258, 
259. 

Tartaric  add,  414. 

Taxaceae,  212. 

Taxic  movements,  431. 

Taxies,  432,  446. 

Taxineae,  212. 

Taxodineac,  220. 

Taxus,  213;   microsporophyll,  214. 

Telegraph  plant,  y  f. 

Teleutospore,  82,  83. 

Temperature,  and  death,  4S3  ;  and  nastic 
curvatures,  44.5 ;  and  photosynthesis,  .572 ; 
and  transpiration,  3.->o. 

Tendrils.  469. 

I  ension  of  tissues,  t-'5. 

1  erpenes,  11  ;. 

Testa,  [83,  id'),  205,  209,  213. 

Tetanus,  432,  433- 

Tetracyc  lit  Bowers,  281. 

Tetraspores,  55,  59,  60. 

Thallophytes,  1. 

Thelephorales,  87. 

Theobromin,  415. 

Thermotropism,  479. 

Thigmotropism,  y»>. 

Thuja,  archegonium  complex,  222. 

ris,  habit  and  sporangia,  142. 

Toadstools,  87. 

Tobai  co,  Bower, 

Tolypothrix,  9. 

Tone,  434. 

Tooth  fungi,  88. 

Torreya,  archegonium,  215;  embryo,  218;  fe- 
male gametophyte,  216;  fertilization,  2/7; 
male  gametophyte,  217;  microsporophyll, 
21 1;  strobilus,  si  ;.  21  /. 

Tr.il.e.  11I  u 

Tracheae,  .|i.      /;.    ;i-.  .;/;. 


Trai  lieids.  1  50,  220.  241,  .■//.   I  ; 
nili.i,  r.M.I  tips,  247. 

["ran  lot  ation  ..1  1 1,  (88  ;  rhyihmi. . 

Transmission  of  stimuli,  430. 
Transpiration,  321,  323;    factors,  320;    and 
growth,  ,s -'*>  ;    and  salts,  325. 
il  ropism,  472. 

Tree  ferns,   [56. 

Trehalase,  400. 
Tremellales,  86. 

ne,  56,  57,  80. 
Trichomanes,  sporangia,  156. 

Triple  fusion,  270. 
Tropaeolum,  nectary,  339. 

Tropic  movements,  431. 

Tropisms,  432,  458. 

True  mosses,  1 15. 

Truffles,  7 \. 

Twiners,  4(17. 

Tube  cell,  [99. 

Tuberales,  74- 

Tubifloralcs,  282. 

Tumboa,  embryo,  236  ;  female  gametophyte, 
234;  "flowers,"  230;  habit,  229;  strobi- 
lus, 22Q. 

Turgidity,  308. 

Turgor,  309;  and  growth,  310  ;  movements, 
451,  457  ;   rigidity  from,  310. 

Ulothrix,  24,  25. 

Ulva,  26. 

I'mbellales,  280. 

Umbelliferae,  280. 

Uncinula,  75,  76. 

Uredinales,  82. 

1  redo,  84. 

Uredospore,  82. 

Urostyla,  movement  of  cilia,  446. 

Use  and  disuse,  284. 

Ustilaginales,  81. 

Vacuole,  297. 

Yasi  ular  anatomy,  Bennettitales,  185 ; 
Coniferales,  219;  Cordaitales,  203  ;  Cyca- 
dales.  102,  ;,;/,•  Cycadofilicales,  t8i,  182; 
Dicotyledons,  242,  245;  Equisetum,  145; 
Filicales,  156,  159,  /<•>",  161,  162;  Ginkgo, 
207;  Gnetales,  229;  Isoetes,  138;  Lyco 
podium,  124,  125;  Monocotyledons,  244, 
245;  Ophioglossales,  149;  root,  247,240; 
Ha,  /  y. 

Vaucheria,  14, 

Vegetative    multiplication,    algae,    0,     10; 

mOSSt  ■-,.  1  in. 

Velum,  Norte-,  1  jo;  mushroon 
\'ieia,  geotropit  rool  curvature,  f>i. 
\'oU  a,  mushrooms,  88. 


INDEX 


Yolvocales,  15. 
Vol  vox,  1 8,  iq. 

Wastes,  412. 

Water,  ascent,  349 ;  capillary  ascent,  314; 
continuity,  301  ;  and  death,  483  ;  entry, 
316;  exudation,  332;  immigration,  308; 
influx,  325  ;  loss,  331  ;  migration  into  roots, 
314;  movement,  341;  and  plants,  299, 
311;  raw  material,  366;  relations,  301; 
soil,  313  ;  solvent,  303. 

Water  ferns,  170. 

Water  molds,  63. 

Water  proofing,  317,  318. 

Weber's  law,  448. 

Weismannism,  200. 

Welwitschia,  228. 

Wheat,  aleurone  grains,  392. 

Wheat  rust,  82,  83,  84,  85. 

White  rust,  65. 

Witch  brooms,  71. 

Wood,  heart  and  sap,  347. 


Xanthophyll,  367. 
Xenia,  271. 
Xylaria,  77. 

Xylem,  124,  241,  342,  343,  344,  345;  water 
path,  347. 

Yeast,  70  ;  fermentation,  410. 

Zamia,  embryo,  200,  201  ;  habit,  193;  sta- 
men, 195;  stem  section,  104;  strobilus, 
196. 

Zonal  development,  254. 

Zoospore,  16,  22,  23,  25,  26,  28,  2Q,  30,  32,  34, 
35,  45- 

Zygnema,  38,  40. 

Zygnemaceae,  38. 

Zygomorphic  (lowers,  256. 

Zygomycetes,  67. 

Zygospore,  16,  17,  22,  23,  25,  3S,  39,  40. 

Zygote,  16. 

Zymase,  410. 


ADVERTISEMENTS 


PLANT      LIFE      A  N  L> 
PLANT      USE  S 

By  JOHN   GAYLORD   COULTER,   Ph.  D. 

5  i. 20 


AN  elementary  textbook  providing  a  foundation  for  the 
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about  the  fundamentals  of  plant  life  and  about  the  relations 
between  plants  and  man.  It  presents  as  fully  as  is  desirable 
for  required  courses  in  high  schools  those  large  facts  about 
plants  which  form  the  present  basis  of  the  science  of  botanv. 
Yet  the  treatment  has  in  view  preparation  for  life  in  general, 
and  not  preparation  for  any  particular  kind  of  calling. 

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INTRODUCTION    TO   POLITICAL 
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I2.5O 


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and  on  citizenship  and  nationality. 

^j  Before  stating  his  own  conclusions  the  author  gives  an  im- 
partial discussion  of  the  more  important  theories  of  the  origin, 
nature,  and  functions  of  the  state,  and  analyzes  and  criticises 
them  in  the  light  of  the  best  scientific  thought  and  practice. 
Thus  the  pupil  becomes  familiar  with  the  history  of  the  science 
as  well  as  with  its  principles  as  recognized  to-day. 


AMERICAN     BOOK    COMPANY 


